Differential and AT fluid changes

[Snowbound]

Does anyone know the reason for the recommendation to change differential fluids and ATF at 30K mile intervals (for non towing outbacks)?

Is there science that shows 30K is when those fluids have lost proper lubing power?

[1 Lucky Texan]

????

I think the schedule indicates they should be inspected. (AT at 30 - then 90)

what car/year model?

Is this the stealer... - um - dealership recommending that service?

[Snowbound]

2009 outback. The manual only provides inspection intervals for differential and AT fluids unless the vehicle is used for towing.

Searching these forums will result in suggestions to change these fluids at 30K miles. But why is 30K the magic number? Why not 20K, 50K or 100K?

[1 Lucky Texan]

I think part of the thought behind inspecting stuff is to just get ahead of potential chronic prblems like leaks or overheated/dark fluids.

Some people do change fluids earlier than the schedule suggests - perhaps they are running in a severe duty scenario or trying to increase gas mileage with a suposedly superior lubricant.

I don't think Subaru schedules for this stuff is much different than other cars so, I suspect the automotive and lubrication fluid industries have created a 'best guess' on some of these things. I know people have used blackstone labs and similar services to try to determine the best schedule for engine oil changes. You might dig around at bobistheoilguy.com for info about testing other fluids.

[Snowbound]

The problem is there isn't a schedule for changes for these fluids.

[1 Lucky Texan]

I'm not sure what your complaint is.

If I inspect (per schedule) my AT fluid at 90k miles and find it is dark and smells bad, I probably would do at least a partial change.

If I frequently drive my vehicle on dirt roads and across creeks - I might decide my rear differential fluid is being abused by increased eposure to dust and moisture and, as a preventative, I might change it on some self-selected schedule. Or, early in the car's life, I might switch to a superior lubricant hoping it will be less prone to degradation from extreme service.

I once owned a Mitsubishi whose manual suggested extreme duty was frequent operation in temps above 90* (yeah - NINETY degrees)

that's 1/3 of the year where I live. I changed the AT fluid on the inspection cycle.

Also, on used vehicles, one does not always know what conditions the PO operated it in or if they replaced fluids - perhaps with something inappropriate, so, changing the fluids gives you, the new owner, a known 'base' to start from.

Other people maybe do it because they enjoy tinkering with their car. Folks who do their own service fall into about 2 categories; folks that really NEED to save money, and those that have car maintenance as a sort of hobby. Sure, there's some overlap. And folks that post/participate on the Forums are often pretty hardcore gearheads and they are often looking for ways to maximize the lifespan and reliability of cars so, it's easy to see why you may find some folks recommending accelerated schedules for some services. Also, manufacturers don't want their cars to appear to be expensive to maintain and may - um - 'stretch' some services out a little. But, if you feel someone, or even 3-4 people, are recommending some maintenance that you feel is wrong, just ignore that advice and stick with the manufacturer's schedule - MOST important during the warranty period of course.

Cars are full of compromises and so is their maintenance.

[RobLog]

When the car leaves the factory is the only time it will have 100% new ATF. Given that a drain-and-fill only replaces about 40% of the ATF, doing it every 30k ensures that the fluid is periodically refreshed and doesn't get too stale or too contaminated.

I once sent in an ATF sample from my F150 with an E4OD trans to Blackstone. Their report stated:

"Universal averages show typical wear metals from this type of transmission after about 26,500 miles on the oil. This oil has been in use about 80,000 miles, so it makes sense that all the wear metals are very high. The viscosity was low and insolubles were high, and this (in addition to the high metals) is a good sign that this oil has been in use way too long. But you'll probably find nice improvements once you change it (and we do suggest changing it now)."

This was on a lightly driven vehicle, i.e., no severe use.

I no longer submit samples, but do a d-n-f on the subie once a year and on the truck every 2-3 years, regardless of mileage (and it's low). If one DIYs it, it's not expensive, at all. 4-5 quarts of Valvoline Maxlife ATF for the subie is about $30 ... cheap insurance.

EDIT: Just wanted to add that the F150 shifted just fine even with the worn out, contaminated oil and color was only a little darker than fresh, though it did shift smoother after the change (15 quarts of Mobil-1 synthetic ATF ... $$$ ouch). IMO, a visual inspection is close to useless ... and an analysis costs about the same as a DIY d-n-f, so one might as well d-n-f on a reasonable schedule.

[1 Lucky Texan]

Originally Posted by RobLog

When the car leaves the factory is the only time it will have 100% new ATF. Given that a drain-and-fill only replaces about 40% of the ATF, doing it every 30k ensures that the fluid is periodically refreshed and doesn't get too stale or too contaminated.

I once sent in an ATF sample from my F150 with an E40D trans to Blackstone. Their report stated:

"Universal averages show typical wear metals from this type of transmission after about 26,500 miles on the oil. This oil has been in use about 80,000 miles, so it makes sense that all the wear metals are very high. The viscosity was low and insolubles were high, and this (in addition to the high metals) is a good sign that this oil has been in use way too long. But you'll probably find nice improvements once you change it (and we do suggest changing it now)."

This was on a lightly driven vehicle, i.e., no severe use.

I no longer submit samples, but do a d-n-f on the subie once a year and on the truck every 2-3 years, regardless of mileage (and it's low). If one DIYs it, it's not expensive, at all. 4-5 quarts of Valvoline Maxlife ATF for the subie is about $30 ... cheap insurance.

good post, i wasn't sure labs would inspect fluids other than engine oil.


I do think it falls into the category of cheap insurance. Do some people change fluids prematurely? I'm sure they do - just like with engine oil that has no analysis.

I could see a new car going a few years or 60-100K miles, then begin yearly d-n-f . And I feel it's unreasonable to expect a southern Arizona car to use the same schedule as a car in -say - Minneapolis, for all it's fluids.

[Snowbound]

Originally Posted by 1 Lucky Texan

If I frequently drive my vehicle on dirt roads and across creeks - I might decide my rear differential fluid is being abused by increased eposure to dust and moisture and, as a preventative, I might change it on some self-selected schedule.

How does dust get into the rear differential fluid?

[RobLog]

How does dust get into the rear differential fluid?

AFAIK, all differentials are vented to avoid the buildup of pressure. I know my 97 has one. See item #7 in the link.
http://opposedforces.com/parts/legac...al_individual/