2018 Subaru Outback Front and Rear Differential - 125k miles - Subaru 75w-90 HP Gear Oil

[RevvCats]

I changed the factory diff oil in my parents 2018 outback and figured it would be interesting to send samples out for analysis to see how well the oil held up after 125k miles and to see the differences between the front and the rear differentials.

Sample 1 is the front diff, 2 is the rear diff

There’s some VOA info posted about Subaru High Performance gear oil from 10 years ago. If the specs are the same this oil sheered down a fair bit in both the front and the rear, slightly more in the front.

 

[Slick3914]

Both of those look pretty good for 125,000 miles.

 

[fantastic]

That is some thick gear lube and should still serve you well. Thanks

 

[Glenda W.]

Thanks for this!! Looks good for the mileage.

 

[Fa24wrx]

So you're saying changing my front/ rear diff every 25k on my 22 wrx manual transmission is good enough?

 

[Rob_Roy]

So you're saying changing my front/ rear diff every 25k on my 22 wrx manual transmission is good enough?

Likely way overkill. IMO 80% if the benefit is achieved on the first change.

 

[Fa24wrx]

Likely way overkill. IMO 80% if the benefit is achieved on the first change.

I've only done it the one time. Still at 37k miles

 

[BISCUT]

I've found the front diff on my 17 Outback appears to have a much harder life than the rear diff. I did drain/fill the front and rear about 25k and again at 125k. The rear was similar to new, while the front was a lot worse for the wear. Visual only but the differences were stark.
I'll be at 200k soon and will do them again for the last time until she dies!

 

[Fa24wrx]

I imagine having a manual transmission share the fluid with the front diff majes a difference on when to change that fluid.

 

[BISCUT]

I imagine having a manual transmission share the fluid with the front diff majes a difference on when to change that fluid.

Front diff shared with CVT and it shows.

 

[Fa24wrx]

Front diff shared with CVT and it shows.

Which one do you think would be more "stress" on the fluid?

 

[RevvCats]

Front diff shared with CVT and it shows.

The two things are bolted together but the CVT used a different fluid from the front diff

 

[RevvCats]

Both of those look pretty good for 125,000 miles.

Yeah this stuff came back looking better than I expected. The front diff oil came out looking new and the rear diff oil was significantly darker but that looks to be from the higher amount of iron wear.

This car sees a lot of highway driving so most of those 125k were steady cruising miles. My sister has a crosstrek that lives a hard life of stop and go city driving. Less mileage, ~50k, but when I see her next I’m thinking of changing her diff oil to see how that stuff has held up.

 

[BISCUT]

The two things are bolted together but the CVT used a different fluid from the front diff

I was way off on that one!! Thanks for correcting it. I started a thread on my last diff drain/fill. Here was the explanation @SubieRubyRoo


Eh, this is somewhat true, but certainly not anywhere near “99%” (max is 90% FWD unless the fuse is pulled to disable the center diff) until the car is at freeway speeds. The center differential is electronically controlled, and at low speeds is directly linked to maintain a near 50/50 distribution especially when there is a speed difference between the front and rear diff speeds. As vehicle speed increases, the center diff progressively “slips” and will eventually get to 90/10 front/rear on all but the models with the Sport center diffs. The WRX STi is adjustable and does not use the same progressive scheme.

To OP, yes, I think the front diff experiences more use/wear because when the steering is actuated, the front wheels spin at significantly different speeds due to the Ackerman angle. I’ve seen the same thing you did where front diffs are much darker and more metal “fluff” on the magnet. I’d either do UOAs to check fluid, or change the front diff at half the OCI of the rear.

The other thing that most don’t know is that nearly all Subaru rear diffs other than WRX are open; the Legacy/Outback had “sealed” limited slips but I don’t know the latest gen. The one on my ‘05 Outback cannot change the LS diff fluid itself; you can only change the fluid that contacts the gearset- the diff itself is completely sealed and unserviceable.

 

[RevvCats]

To OP, yes, I think the front diff experiences more use/wear because when the steering is actuated, the front wheels spin at significantly different speeds due to the Ackerman angle. I’ve seen the same thing you did where front diffs are much darker and more metal “fluff” on the magnet. I’d either do UOAs to check fluid, or change the front diff at half the OCI of the rear.

It was actually the rear diff oil that was significantly darker, the front diff oil came out looking pretty clean. The amount of iron wear was double that of the front diff. The front gear oil had sheered slightly more. If the voa from 10 years ago is still accurate the Subaru oil starts off at 15 cSt @ 100C

 

[The Critic]

Are you sure this is the factory fill? 2018 Outback owners manual shows the front diff should have been factory filled with Extra MT, which is a 10cst (ish) fluid.

It looks like both differentials had 75W90 in them.

 

[RevvCats]

Are you sure this is the factory fill? 2018 Outback owners manual shows the front diff should have been factory filled with Extra MT, which is a 10cst (ish) fluid.

View attachment 268955

It looks like both differentials had 75W90 in them.

Unless a shop did it without saying so or charging for it, that’s the factory fluid. I thought MT fluid was only used in the front diff with manual transmissions where the fluid isn’t separated.

 

[The Critic]

Unless a shop did it without saying so or charging for it, that’s the factory fluid. I thought MT fluid was only used in the front diff with manual transmissions where the fluid isn’t separated.

I highly doubt that this was the factory fill. The amount of wear metals in your sample is very low for 125K, way too low. If this vehicle was every sold a 60/120k service at a dealer, the differential fluid was done.

This is a similar Subaru differential with 25K on the factory fill:
https://www.subaruxvforum.com/threads/used-oil-analysis-front-and-rear-differentials-at-25k.63665/

 

[RevvCats]

I highly doubt that this was the factory fill. The amount of wear metals in your sample is very low for 125K, way too low. If this vehicle was every sold a 60/120k service at a dealer, the differential fluid was done.

This is a similar Subaru differential with 25K on the factory fill:
https://www.subaruxvforum.com/threads/used-oil-analysis-front-and-rear-differentials-at-25k.63665/

Car was bought new off the lot. I don’t know what to tell you because a front+rear gear oil change was never paid for and I don’t think there’s a gang of oil elves running around upstate ny changing peoples gear oil at night.

My guess is the style of driving was different. This car sees a lot of highway driving. My sister had a crosstrek that’s almost entirely city driven and if I ever have the chance I’ll get samples from that cars front and rear diff.

 

[sds911]

I highly doubt that this was the factory fill. The amount of wear metals in your sample is very low for 125K, way too low. If this vehicle was every sold a 60/120k service at a dealer, the differential fluid was done.

This is a similar Subaru differential with 25K on the factory fill:
https://www.subaruxvforum.com/threads/used-oil-analysis-front-and-rear-differentials-at-25k.63665/

Ah yes. I remember that UOA well. My old crosstrek hybrid. My use was mostly highway within CA and with mountain trips from CA to CO (Vail Pass and Berthoud Pass). I think I put 40k miles on the suv pretty quickly. Sold it and ended up with a bmw 3 series diesel awd wagon.

And my thought exactly when I saw the low wear numbers on a 125k run…I like the concept of a diff oil fairy or elves though, cause gear oil stinks. The crosstrek uoa convinced me to dump factory fill early going forward. Did it at 15,800 on my lexus gx. Just posted my UOA of second fill on Gx with Motul Gear 300 tonight with 39k on Motul.

See a lot of the crosstreks now that I’m back in Colorado. And every third car is a Subaru (jk).