What fluids should you change on your car and how often?
Changing fluids on a car has long been viewed as essential to long-term car care. However, in recent years, changes to automotive technology have made such fluid changes more complex, and in some cases unnecessary or not as often as previously required.
Such fluids include, but are not limited to, engine oil, engine coolant, brake fluid, power steering fluid, transmission fluid, differential fluid, and transfer case fluid. Some, such as engine oil, need to be changed fairly often. Others may never be changed, or changed once or twice in the life of the vehicle.
What fluids should you change in your car and how often? The following is a only a general discussion. Consult your owner's manual or your manufacturer for specific information on your make and model car. As will be discussed in more detail below, today, more than ever before, specific cars have specific fluid needs, both in type and change intervals. There is no way to address all types in any one article.
1. Economic Interests: The first thing to understand is the economic interests involved in fluid changes. Automakers like to recommend long intervals between fluid changes for a number of reasons. Longer intervals means the maintenance costs can be kept to a minimum. In addition, since many cars now come with "free maintenance" for the first few years, eliminating fluid changes keeps costs low. So the car makers are going to recommend the longest change intervals possible - possibly even never changing some fluids.
Not too long ago, many car makers shipped cars with "break-in oil" in the engine, transmission, and differential. After 500 miles (!) the car would be brought back to the dealer for a complete fluid change regimen. Today, the same car makers are telling you not to come back for 15,000 miles or more - even for an engine oil change!
Fluid makers obviously want you to change fluids more often than that. So Jiffy Lube says to change your oil every 3,000 miles, while car manufacturers say you can go 5,000 to 7,500 miles - or perhaps as long as 15,000 miles or more, with synthetics. And fluid makers shill car boards with postings, suggesting that in order to make a car last 100,000 miles, you need to change the transmission fluid every 30,000 miles.
Both sides in this debate have vested economic interests and thus their recommendations should be considered in view of what they are trying to sell. In general, I view the car maker's recommendations as the longest interval you should consider, while the fluid maker's paranoid recommendations being perhaps overly short. Improvements in engine oils and filters mean that engine oil change intervals, for example, can be extended far beyond those used in the past.
2. Changes in Technology - longer fluid intervals: It is true that car technology has changed dramatically in the last few decades. With the advent of fuel injection and modern emissions controls, cars run cleaner than in the days of the carburetor. So excess fuel is not dumped into the crankcase at startup, causing excess ring wear. As a result, engines last longer, oil runs cleaner, and doesn't need to be changed as often.
In addition, oils have improved in quality and technology. Synthetic oils in particular, can last a long time without losing viscosity or lubricity. Keep this in mind when considering oil change intervals.
3. Changes in Technology - Diversity: Not long ago, cars had three or four fluids, and all cars used the same fluid types and brands. Engines ran on 30-weight oil in the summer or perhaps 10W30 in the winter. Manual transmission and differential fluid was straight 90 weight gear oil. Automatics used one of two types of fluids, Mercon, or Dextron. And brake fluid and radiator coolant (the latter mixed with tap water) were pretty standard.
Today, we have a host of manual and automatic transmission fluids to choose from. And some manual transmissions use automatic transmission fluid, to boot. And new SMG Automatics may use manual transmission fluids. Differential oils come in a number of viscosities, and limited slip differentials may require special fluids. Engine coolant has become more complicated than "Just Prestone", as many manufacturers recommend specific fluids for their aluminum high-tech engines, and please, mix with distilled water, not tap! Brake fluids come in a number of different formulas as well.
That's why it does pay to check with your manufacturer first, before throwing 90 wt. gear oil in your transmission. It is possible you may ruin it. The transmission may take one of five "MT" type fluids, or possibly Automatic Transmission Fluid (ATF). It is more confusing today, than ever.
4. Lifetime Fluids: In the 1960's, many fluids were "lifetime" only because no one kept cars that long. At 80,000 miles, the typical American car was pretty washed up, and chances are, it went to the junkyard with the original fluid in the differential, transmission, brakes, power steering, and possibly even the radiator.
Today, we are more proactive about changing such fluids, but we also expect cars to run 150,000 miles or more - possibly as long as 300,000 miles, before they are junked. And transmissions and differentials today are lighter weight and less robust than the cast-iron transmissions with forged gears of the past. Today, lightweight powdered metallurgy and nodule cast iron gears are more the norm. And fluid changes often extend the life of such components.
But some manufacturers are touting "lifetime" fluids again. These specially formulated fluids are expensive to buy ($25 to $50 a quart, in some instances) and in some cases, dealers will refuse to change such fluids, on the grounds that they are "lifetime" fills. However, it remains to be seen what a "lifetime" comprises - 100,000 miles? 150,000? 200,000? Since many of these "lifetime" fluids were only introduced a few years ago, no one really knows.
Whether you want to go along with the "lifetime" recommendation is up to you. With the price of the fluids, changing the often is prohibitively costly. I compromised on one car, changing the "lifetime" fluids at the 100,000 mile mark. We'll see how that works out.
5. Flushing Brake Fluid: Don't ask Jiffy Lube to do this, as they will flat-out refuse to touch brake fluid. In the old days, they would "top off" brake fluid. Bad idea. If your brake fluid is low, have the car towed to a mechanic, as there is a leak in the system, and it is only a matter of time until the pedal goes to the floor and you crash into something. After a few lawsuits, Jiffy Lube decided that not touching brakes at all was a swell idea.
BMW recommends flushing brake fluid every 24,000 miles or two years, on the theory that since the fluid is hygroscopic, it absorbs water and thus will cause rust in the system over time. I have been doing this regularly (not hard with a bleeder, and provides an opportunity to rotate the tires as well). But I have never heard of this being recommended with other makes and model cars.
Brake fluid does change color over time, with most clear or gold fluid turning rusty brown eventually. So I suppose regular flushes could make calipers and other components last longer. Again, check your owner's manual or manufacturer's recommendations to be sure. And have a qualified mechanic do this work if you are not trained in it. Brakes are nothing to mess around with.
Again, a number of cars probably go to the grave with original fluid in them. You'd be surprised.
6. Engine Coolant: This is another area where frequent flushes are recommended by some makers (BMW) while others (GM) tout their "lifetime" Dexcool. Again, this once simple area has become complicated.
If you are old enough, you remember when "permanent antifreeze" (ethylene glycol) first became available. It was "permanent" in that the old type of fluid (alcohol) evaporated out of the system over time and had to be replaced annually. And many people back then took the "permanent" moniker to heart - never changing their anti-freeze over the life of the car.
And we now call it "coolant" not "anti-freeze". Ethylene glycol and its cousins have a higher boiling point that water, so not only do they protect against freezing, they keep the car from boiling over, as well.
Again, BMW recommends flushing this every few years and replacing with new BMW brand coolant (which is blue) and distilled water in a 50:50 mix. I follow these instructions only because the cars are so darn expensive, and the cost of the coolant and distilled water (the latter being only 99 cents a gallon) is reasonable enough.
At the other extreme, GM uses it's Texaco Dexcool in all their cars and recommends never changing it - ever. The only problem with Dexcool is that if the fluid level gets low, and air gets into the system, it can solidify into a red, sand-like mass and overheat the engine. So you have to make sure the level never goes low. And if it does, make sure you top off with more Dexcool (which is red) and not mix it with some other coolant.
Car engines are mostly made of aluminum, and radiators have switched from all-brass to aluminum and plastic. Many people claim the high-phosphate content of old Prestone-type green coolants made such plastic brittle. And others claim that hard water from the tap could cause electrolytic problems with aluminum engines, or cast iron engines with aluminum heads (or vice versa). Prestone now has a yellow cooolant that they claim is "safe for all engines". I use it in my truck. Not in my BMWs.
Adding to all this confusion of Red, Blue, Yellow, and Green coolants, is the presence of things called "water wetters" which sound like the ultimate snake oil. Should you use these? Are they bogus? It is your choice. They do apparently work, by reducing the surface tension in the coolant. Ford *required* water wetter in their 7.3 liter Powerstoke Diesels a few years back. It seems that the shock wave produced by the combustion caused cavitation to occur in the cooling jacket in the #7 cylinder. This cavitation eventually ate through the cylinder wall, causing engine failure. Water wetter, for whatever reason, solved the problem. If you flushed your radiator in your Ford diesel and didn't use water wetter, well, you just voided the warranty.
Such incidents illustrate how esoteric fluid changes have become, and how using the "same old fluids my Daddy used" can really cause a lot of trouble for you down the road.
7. Engine Oil: This is an area where you can start an argument among normally reasonable men. Many men have stronger opinions on oil changes than they do on religion. And I guess I will have to be a total heretic here, as frankly, so long as your engine oil is occasionally changed, it really makes no difference.
Once you are over being outraged over my last statement, sit down, take a deep breath, and listen. Modern car engines run so much cleaner than days of old. Carburated cars would dump a load of gas down the intake whenever you started them cold. This would wash down the cylinder walls, causing a lot of wear on startup. Most car engines were burning oil by 80,000 miles back then.
And the gasoline in the oil didn't help it. And since the engines ran "dirtier" they put more dirt in the oil as well. On top of this, most engine oils in the 1960's and earlier were pretty primitive compared to today.
One huge mistake I see people make is over-maintaining a car in one area. And the classic here is in over-changing oil. Young men in particular, are convinced they will make their car last "forever" by changing the oil more often. I know I did! What young men fail to take into account is that they change cars on the average of every 2-5 years, if not more frequently.
I went into this in detail in an earlier posting, so I won't repeat all of that here. Suffice it to say that the "secret" of a high mileage car is no secret. Drive it more! Cars that go 300,000 miles or more before blowing an engine are generally those driven a lot in a short period of time. As a result, they tend to have a lot of highway miles and thus little wear.
Changing the oil every 3,000 miles isn't going to make your car last forever. Chances are, something else will break before the basic engine components wear out. Ring jobs just aren't that common anymore. An overheating incident or expensive transmission repair are far more likely to send a car to the junk yard than ring wear. That, or an accident where the damage exceeds the book value.
So yes, change your oil regularly. But don't obsess about it or think you can "beat the reaper" by going to an extreme regimen. Because all you're really doing is increasing your maintenance costs with little or no benefit to you.
8. Transmission Fluid: Again, many manufacturers are now claiming these fluids are "lifetime". And in the past, they were often not changed anyway. But of course, in the past, cars didn't last as long.
An old Muncie 4-speed might go to the junkyard with its original 90-wt inside it. But today's lighter weight 5- and 6-speeds might benefit from a change during their operating life.
How often? Read your owner's manual. And again, remember there are a lot of fluids out there now, and the wrong one could cause more harm than good.
Automatics are getting trickier as well. BMW stopped putting dipsticks on its traditional torque-converter automatics years ago (and today, they have taken the dipstick off the engine as well!). So there is no way to sample the bright red ATF and see if it has burnt to a dirty brown (which should be changed immediately). Also, transmissions can leak, which can cause severe problems if the fluid gets low enough for the transmission to overheat. If your car is leaving red drips in the parking lot or garage, check the fluid level!
Among RV'ers, the philosophy is to change transmission fluid, even if a manufacturer says not to, or to do it infrequently. RV's and tow vehicles have a hard life, and transmissions take the brunt of a lot of the load - heating up and burning the fluid. Changing the fluid regularly (40,000 miles or so) under severe conditions like that, is often a good idea.
But for most people who are using their cars for general transportation, and not continuous towing, such extremes are probably not necessary.
9. Differentials: Again, in the past, this was a fluid that stayed with the car until it went to the junkyard. Today, many enthusiasts like to replace this fluid on the grounds that it may have gone south.
Differentials do heat up, as the hypoid gears have a lot of sliding friction. Limited-slip differentials (rapidly becoming a thing of the past, thanks to traction control) generate a lot of heat when their clutch packs engage. Some differentials, like the one on my BMW M Roadster, have heat sink fins to cool them down - that much heat can be generated!
Again, follow your manufacturer's instructions, which likely will say to leave this alone. The only exception would be, again, if you are using the car for towing, racing, or otherwise causing additional stress on it.
And use the correct fluid as well. Again, the days of the old 90 weight gear oil are largely past us. Today, multi-viscosity oils and limited-slip oils are among the many choices, including some bespoke oils that some manufacturers recommend.
10. Power Steering Fluid: Many people neglect this fluid, assuming that as simple hydraulic fluid, it never wears out. This may have been true in the old days, but I notice in some cars, that the power steering fluid (which is usually automatic transmission fluid, check your manual for correct type) tends to turn from its bright red when new, to a dirty black after a few years. A simple oil extraction pump can remove most of the fluid from the system, which can be replaced easily at the reservoir.
11. Cost/Benefit Analysis: If all of this sounds like a lot of work to you, you may be right. Replacing all of the fluids listed above could easily cost hundreds and hundreds of dollars, for the materials and labor, particularly if you have a vehicle that requires "special" fluids (read: BMW). Whether it is worthwhile to do so is entirely up to you.
Many folks like to trade-in cars when they reach a certain age, and if this fits your profile, then changing some of these fluids might not make much sense, unless it is provided as part of the free maintenance, or is required to maintain the warranty. If you sell your car after four years, there is little point in flushing even the radiator, much less the transmission or differential. The second owner might wish you did, but that is his problem.
Even if you plan on keeping a car for 100,000 or 150,000 miles, many of these fluids might not require changing. BMW's "lifetime" transmission fluid, for example, seems more than capable of going 100,000 to 150,000 miles, according to some users. Whether it will last 300,000 miles remains to be seen.
Like anything else, you have to do a cost/benefit analysis. I saw one person on a car board say that he changed his transmission fluid every 30,000 miles. Now, either he has been taken in by a fluid manufacturer, or he works for one and is shilling. Because unless you are towing an 8,000 lb trailer behind your car, chances are, that is overkill. And even with an 8,000 lb. trailer, you could probably go to 40,000 miles or more.
There is such a thing as over-maintaining a car. Replacing 100,000 mile spark plugs every 50,000 miles isn't going to make your car run better. It will just make you a little poorer and waste a little of your time. Similarly, replacing air filters before their time really doesn't do much for you, unless you just drove through a dust storm. Overly-frequent fluid changes could also be a waste of money and time, with no real improvement in service life or performance.
The decision is, of course, up to you. Look at the manufacturer's recommendations and then realistically consider how long you expect to keep the car and how you plan on using it. Then decide on what sort of fluid change regimen makes the most amount of sense for your vehicle.
Such fluids include, but are not limited to, engine oil, engine coolant, brake fluid, power steering fluid, transmission fluid, differential fluid, and transfer case fluid. Some, such as engine oil, need to be changed fairly often. Others may never be changed, or changed once or twice in the life of the vehicle.
What fluids should you change in your car and how often? The following is a only a general discussion. Consult your owner's manual or your manufacturer for specific information on your make and model car. As will be discussed in more detail below, today, more than ever before, specific cars have specific fluid needs, both in type and change intervals. There is no way to address all types in any one article.
1. Economic Interests: The first thing to understand is the economic interests involved in fluid changes. Automakers like to recommend long intervals between fluid changes for a number of reasons. Longer intervals means the maintenance costs can be kept to a minimum. In addition, since many cars now come with "free maintenance" for the first few years, eliminating fluid changes keeps costs low. So the car makers are going to recommend the longest change intervals possible - possibly even never changing some fluids.
Not too long ago, many car makers shipped cars with "break-in oil" in the engine, transmission, and differential. After 500 miles (!) the car would be brought back to the dealer for a complete fluid change regimen. Today, the same car makers are telling you not to come back for 15,000 miles or more - even for an engine oil change!
Fluid makers obviously want you to change fluids more often than that. So Jiffy Lube says to change your oil every 3,000 miles, while car manufacturers say you can go 5,000 to 7,500 miles - or perhaps as long as 15,000 miles or more, with synthetics. And fluid makers shill car boards with postings, suggesting that in order to make a car last 100,000 miles, you need to change the transmission fluid every 30,000 miles.
Both sides in this debate have vested economic interests and thus their recommendations should be considered in view of what they are trying to sell. In general, I view the car maker's recommendations as the longest interval you should consider, while the fluid maker's paranoid recommendations being perhaps overly short. Improvements in engine oils and filters mean that engine oil change intervals, for example, can be extended far beyond those used in the past.
2. Changes in Technology - longer fluid intervals: It is true that car technology has changed dramatically in the last few decades. With the advent of fuel injection and modern emissions controls, cars run cleaner than in the days of the carburetor. So excess fuel is not dumped into the crankcase at startup, causing excess ring wear. As a result, engines last longer, oil runs cleaner, and doesn't need to be changed as often.
In addition, oils have improved in quality and technology. Synthetic oils in particular, can last a long time without losing viscosity or lubricity. Keep this in mind when considering oil change intervals.
3. Changes in Technology - Diversity: Not long ago, cars had three or four fluids, and all cars used the same fluid types and brands. Engines ran on 30-weight oil in the summer or perhaps 10W30 in the winter. Manual transmission and differential fluid was straight 90 weight gear oil. Automatics used one of two types of fluids, Mercon, or Dextron. And brake fluid and radiator coolant (the latter mixed with tap water) were pretty standard.
Today, we have a host of manual and automatic transmission fluids to choose from. And some manual transmissions use automatic transmission fluid, to boot. And new SMG Automatics may use manual transmission fluids. Differential oils come in a number of viscosities, and limited slip differentials may require special fluids. Engine coolant has become more complicated than "Just Prestone", as many manufacturers recommend specific fluids for their aluminum high-tech engines, and please, mix with distilled water, not tap! Brake fluids come in a number of different formulas as well.
That's why it does pay to check with your manufacturer first, before throwing 90 wt. gear oil in your transmission. It is possible you may ruin it. The transmission may take one of five "MT" type fluids, or possibly Automatic Transmission Fluid (ATF). It is more confusing today, than ever.
4. Lifetime Fluids: In the 1960's, many fluids were "lifetime" only because no one kept cars that long. At 80,000 miles, the typical American car was pretty washed up, and chances are, it went to the junkyard with the original fluid in the differential, transmission, brakes, power steering, and possibly even the radiator.
Today, we are more proactive about changing such fluids, but we also expect cars to run 150,000 miles or more - possibly as long as 300,000 miles, before they are junked. And transmissions and differentials today are lighter weight and less robust than the cast-iron transmissions with forged gears of the past. Today, lightweight powdered metallurgy and nodule cast iron gears are more the norm. And fluid changes often extend the life of such components.
But some manufacturers are touting "lifetime" fluids again. These specially formulated fluids are expensive to buy ($25 to $50 a quart, in some instances) and in some cases, dealers will refuse to change such fluids, on the grounds that they are "lifetime" fills. However, it remains to be seen what a "lifetime" comprises - 100,000 miles? 150,000? 200,000? Since many of these "lifetime" fluids were only introduced a few years ago, no one really knows.
Whether you want to go along with the "lifetime" recommendation is up to you. With the price of the fluids, changing the often is prohibitively costly. I compromised on one car, changing the "lifetime" fluids at the 100,000 mile mark. We'll see how that works out.
5. Flushing Brake Fluid: Don't ask Jiffy Lube to do this, as they will flat-out refuse to touch brake fluid. In the old days, they would "top off" brake fluid. Bad idea. If your brake fluid is low, have the car towed to a mechanic, as there is a leak in the system, and it is only a matter of time until the pedal goes to the floor and you crash into something. After a few lawsuits, Jiffy Lube decided that not touching brakes at all was a swell idea.
BMW recommends flushing brake fluid every 24,000 miles or two years, on the theory that since the fluid is hygroscopic, it absorbs water and thus will cause rust in the system over time. I have been doing this regularly (not hard with a bleeder, and provides an opportunity to rotate the tires as well). But I have never heard of this being recommended with other makes and model cars.
Brake fluid does change color over time, with most clear or gold fluid turning rusty brown eventually. So I suppose regular flushes could make calipers and other components last longer. Again, check your owner's manual or manufacturer's recommendations to be sure. And have a qualified mechanic do this work if you are not trained in it. Brakes are nothing to mess around with.
Again, a number of cars probably go to the grave with original fluid in them. You'd be surprised.
6. Engine Coolant: This is another area where frequent flushes are recommended by some makers (BMW) while others (GM) tout their "lifetime" Dexcool. Again, this once simple area has become complicated.
If you are old enough, you remember when "permanent antifreeze" (ethylene glycol) first became available. It was "permanent" in that the old type of fluid (alcohol) evaporated out of the system over time and had to be replaced annually. And many people back then took the "permanent" moniker to heart - never changing their anti-freeze over the life of the car.
And we now call it "coolant" not "anti-freeze". Ethylene glycol and its cousins have a higher boiling point that water, so not only do they protect against freezing, they keep the car from boiling over, as well.
Again, BMW recommends flushing this every few years and replacing with new BMW brand coolant (which is blue) and distilled water in a 50:50 mix. I follow these instructions only because the cars are so darn expensive, and the cost of the coolant and distilled water (the latter being only 99 cents a gallon) is reasonable enough.
At the other extreme, GM uses it's Texaco Dexcool in all their cars and recommends never changing it - ever. The only problem with Dexcool is that if the fluid level gets low, and air gets into the system, it can solidify into a red, sand-like mass and overheat the engine. So you have to make sure the level never goes low. And if it does, make sure you top off with more Dexcool (which is red) and not mix it with some other coolant.
Car engines are mostly made of aluminum, and radiators have switched from all-brass to aluminum and plastic. Many people claim the high-phosphate content of old Prestone-type green coolants made such plastic brittle. And others claim that hard water from the tap could cause electrolytic problems with aluminum engines, or cast iron engines with aluminum heads (or vice versa). Prestone now has a yellow cooolant that they claim is "safe for all engines". I use it in my truck. Not in my BMWs.
Adding to all this confusion of Red, Blue, Yellow, and Green coolants, is the presence of things called "water wetters" which sound like the ultimate snake oil. Should you use these? Are they bogus? It is your choice. They do apparently work, by reducing the surface tension in the coolant. Ford *required* water wetter in their 7.3 liter Powerstoke Diesels a few years back. It seems that the shock wave produced by the combustion caused cavitation to occur in the cooling jacket in the #7 cylinder. This cavitation eventually ate through the cylinder wall, causing engine failure. Water wetter, for whatever reason, solved the problem. If you flushed your radiator in your Ford diesel and didn't use water wetter, well, you just voided the warranty.
Such incidents illustrate how esoteric fluid changes have become, and how using the "same old fluids my Daddy used" can really cause a lot of trouble for you down the road.
7. Engine Oil: This is an area where you can start an argument among normally reasonable men. Many men have stronger opinions on oil changes than they do on religion. And I guess I will have to be a total heretic here, as frankly, so long as your engine oil is occasionally changed, it really makes no difference.
Once you are over being outraged over my last statement, sit down, take a deep breath, and listen. Modern car engines run so much cleaner than days of old. Carburated cars would dump a load of gas down the intake whenever you started them cold. This would wash down the cylinder walls, causing a lot of wear on startup. Most car engines were burning oil by 80,000 miles back then.
And the gasoline in the oil didn't help it. And since the engines ran "dirtier" they put more dirt in the oil as well. On top of this, most engine oils in the 1960's and earlier were pretty primitive compared to today.
One huge mistake I see people make is over-maintaining a car in one area. And the classic here is in over-changing oil. Young men in particular, are convinced they will make their car last "forever" by changing the oil more often. I know I did! What young men fail to take into account is that they change cars on the average of every 2-5 years, if not more frequently.
I went into this in detail in an earlier posting, so I won't repeat all of that here. Suffice it to say that the "secret" of a high mileage car is no secret. Drive it more! Cars that go 300,000 miles or more before blowing an engine are generally those driven a lot in a short period of time. As a result, they tend to have a lot of highway miles and thus little wear.
Changing the oil every 3,000 miles isn't going to make your car last forever. Chances are, something else will break before the basic engine components wear out. Ring jobs just aren't that common anymore. An overheating incident or expensive transmission repair are far more likely to send a car to the junk yard than ring wear. That, or an accident where the damage exceeds the book value.
So yes, change your oil regularly. But don't obsess about it or think you can "beat the reaper" by going to an extreme regimen. Because all you're really doing is increasing your maintenance costs with little or no benefit to you.
8. Transmission Fluid: Again, many manufacturers are now claiming these fluids are "lifetime". And in the past, they were often not changed anyway. But of course, in the past, cars didn't last as long.
An old Muncie 4-speed might go to the junkyard with its original 90-wt inside it. But today's lighter weight 5- and 6-speeds might benefit from a change during their operating life.
How often? Read your owner's manual. And again, remember there are a lot of fluids out there now, and the wrong one could cause more harm than good.
Automatics are getting trickier as well. BMW stopped putting dipsticks on its traditional torque-converter automatics years ago (and today, they have taken the dipstick off the engine as well!). So there is no way to sample the bright red ATF and see if it has burnt to a dirty brown (which should be changed immediately). Also, transmissions can leak, which can cause severe problems if the fluid gets low enough for the transmission to overheat. If your car is leaving red drips in the parking lot or garage, check the fluid level!
Among RV'ers, the philosophy is to change transmission fluid, even if a manufacturer says not to, or to do it infrequently. RV's and tow vehicles have a hard life, and transmissions take the brunt of a lot of the load - heating up and burning the fluid. Changing the fluid regularly (40,000 miles or so) under severe conditions like that, is often a good idea.
But for most people who are using their cars for general transportation, and not continuous towing, such extremes are probably not necessary.
9. Differentials: Again, in the past, this was a fluid that stayed with the car until it went to the junkyard. Today, many enthusiasts like to replace this fluid on the grounds that it may have gone south.
Differentials do heat up, as the hypoid gears have a lot of sliding friction. Limited-slip differentials (rapidly becoming a thing of the past, thanks to traction control) generate a lot of heat when their clutch packs engage. Some differentials, like the one on my BMW M Roadster, have heat sink fins to cool them down - that much heat can be generated!
Again, follow your manufacturer's instructions, which likely will say to leave this alone. The only exception would be, again, if you are using the car for towing, racing, or otherwise causing additional stress on it.
And use the correct fluid as well. Again, the days of the old 90 weight gear oil are largely past us. Today, multi-viscosity oils and limited-slip oils are among the many choices, including some bespoke oils that some manufacturers recommend.
10. Power Steering Fluid: Many people neglect this fluid, assuming that as simple hydraulic fluid, it never wears out. This may have been true in the old days, but I notice in some cars, that the power steering fluid (which is usually automatic transmission fluid, check your manual for correct type) tends to turn from its bright red when new, to a dirty black after a few years. A simple oil extraction pump can remove most of the fluid from the system, which can be replaced easily at the reservoir.
11. Cost/Benefit Analysis: If all of this sounds like a lot of work to you, you may be right. Replacing all of the fluids listed above could easily cost hundreds and hundreds of dollars, for the materials and labor, particularly if you have a vehicle that requires "special" fluids (read: BMW). Whether it is worthwhile to do so is entirely up to you.
Many folks like to trade-in cars when they reach a certain age, and if this fits your profile, then changing some of these fluids might not make much sense, unless it is provided as part of the free maintenance, or is required to maintain the warranty. If you sell your car after four years, there is little point in flushing even the radiator, much less the transmission or differential. The second owner might wish you did, but that is his problem.
Even if you plan on keeping a car for 100,000 or 150,000 miles, many of these fluids might not require changing. BMW's "lifetime" transmission fluid, for example, seems more than capable of going 100,000 to 150,000 miles, according to some users. Whether it will last 300,000 miles remains to be seen.
Like anything else, you have to do a cost/benefit analysis. I saw one person on a car board say that he changed his transmission fluid every 30,000 miles. Now, either he has been taken in by a fluid manufacturer, or he works for one and is shilling. Because unless you are towing an 8,000 lb trailer behind your car, chances are, that is overkill. And even with an 8,000 lb. trailer, you could probably go to 40,000 miles or more.
There is such a thing as over-maintaining a car. Replacing 100,000 mile spark plugs every 50,000 miles isn't going to make your car run better. It will just make you a little poorer and waste a little of your time. Similarly, replacing air filters before their time really doesn't do much for you, unless you just drove through a dust storm. Overly-frequent fluid changes could also be a waste of money and time, with no real improvement in service life or performance.
The decision is, of course, up to you. Look at the manufacturer's recommendations and then realistically consider how long you expect to keep the car and how you plan on using it. Then decide on what sort of fluid change regimen makes the most amount of sense for your vehicle.
