75w140 gear oil OK for Subaru differentials?

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porcupine73

porcupine73

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Hello, I've been running 75w140 GL-5 gear oil in my Subaru front and rear differentials for a few years. It's time to change out the oil, and I'm wondering if I should stick with the 75w140, or maybe go with 75w110 or even 75w90?

The owner's manuals spec 80w90 for the front diff and 75w90 for the rear diff. I just went with the 75w140 to get more protection and also since they're all more than 10 years old and over 100,000 miles I thought they could benefit since they probably have some gear wear. Also I use full throttle a lot but the engines aren't very powerful so maybe it doesn't matter. It's for three soobs, 2000, 1996, 1994.

Other than possible fuel economy loss is there an issue with using 75w140 in there? I am running the Amsoil severe gear right now. Thank you.
 
the hitachi-made rear end in those things is a solid unit. I'd run an x-90 syn. Gained consistent 0.5 mpg after doing so. it doesn't need a 140.

M
 
I mixed 50:50 Valvoline synthetic ATF with SuperTech synthetic 75W140. The differential and transfer case were both drained and refilled with this mix. My Subaru is the Impreza, with a 2.5l normally aspirated, so it does not have the stresses of the turbo charged engine, allowing the use of this lighter oil, which works out to about 75w90, and 100% synthetic. This was done about 5,000 miles ago, and so far everything works well.
At the same time, I drained and refilled the automatic tranny 4 times, the first 2 times with Dex 3, conventional, and the second 2 times with Valvoline synthetic all purpose ATF, so the tranny is running 75% synthetic, and the shifting is PERFECT.
 
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To each there own there Captain, but I would not put ATF in a differential. Not only did you thin the 75W-140, you cut its protection abilities considerably.
 
I have a mix of 75w90 and 85w140 Royal Purple in my diffs (only because I had a limited amount of 75w90). Been this way for 75k? Have not had a problem.

In these applications, I think any quality synthetic will provide good protection...I would probably go with 75w90 next time, just for the fuel mileage (if it actually makes a difference).

97 OBW for the record...
 
Thanks for all the info. I think I may go to 75w90 at least in the winter cars. One thing I did notice with the 75w140 is that the rear diffs howl for a few miles after startup in the cold weather.

I might go 75w110 in the front diffs, since those are going to run hotter, being sandwhiched between the engine and transmission, and the front diff takes the brunt of the power.

I wouldn't be comfortable mixing ATF into my gear oil for the diffs since these are hypoid diffs that specifically require a GL-5 lube for the extreme pressure protection for the gears. The ATF mix might work ok in the manual trans, but even then most manual trans soobs do share the transmission gear oil with the front diff fluid.
 
That is an interesting read about standard transmissions. If either of the diffs in your Subaru has hypoid bevel gears, ATF is the wrong product to use. They spec gear oil for a reason.
 
Originally Posted By: Captain_Klink


also, the Subaru engine is driving ALL four wheels, so the load on each differential is less that, say two wheel drive, which is actually one wheel drive.



And the AWD binding at every corner isn't loading the differentials how???

One thing I did note was that my diff whine disappeared after my first change (to PZ 75w90 syn) at 54k...so the fluid does make a difference. I also have seen almost no metal on the magnetic plug in the front differential since that initial change (factory fluid?).
 
Originally Posted By: deeter16317
Originally Posted By: Captain_Klink


also, the Subaru engine is driving ALL four wheels, so the load on each differential is less that, say two wheel drive, which is actually one wheel drive.



And the AWD binding at every corner isn't loading the differentials how???

One thing I did note was that my diff whine disappeared after my first change (to PZ 75w90 syn) at 54k...so the fluid does make a difference. I also have seen almost no metal on the magnetic plug in the front differential since that initial change (factory fluid?).


MT subies use a limited slip diff in the middle to split power front to rear, so no binding.

AT subies are FWD with a viscous coupling to the rear with electronically-actuated partial (pulsed) lockup when excessive wheel spin occurs, can be binding in high speed sharp cornering.

M
 
I believe you got that backwards...the autos are engaged and disengage in corners in these older models. This is so when/if the duty C solenoid fails, you are failed in AWD.

There is binding in both...it is unavoidable, regardless of the drive train. Whether it is enough to load the diffs (and the oil) is another thing.
 
If you're not a WRX/STI modded to megaHP, then there is no need for the 75w140 that you are using, especially since you are maintaining the gear oil with intervals.

75w90 is probably all you need. 75w110 is a good choice too. Since you mentioned age/mileage, 75w110 is what I would use.

Binding is irrelevant. If the tire is rolling, the diff/bearings are working.
 
Originally Posted By: unDummy


Binding is irrelevant. If the tire is rolling, the diff/bearings are working.



While they might be working, binding places more load on things...that was my point.
 
Originally Posted By: deeter16317
I believe you got that backwards...the autos are engaged and disengage in corners in these older models. This is so when/if the duty C solenoid fails, you are failed in AWD.


hmmm... i really don't think that's true. check some of the other forums. it's a fwd vehicle... cornering/binding occurs b/c the axle rotational differences in sharp corners makes it think there is slippage, hence the system engages the rear. that's a pretty standard system logic across industry, volvo, honda/acura 'yota/lexus, though the mechanisms differ. In fact, IIRC, there were diy mods to this system to apply power to the duty solenoid to provide the full-time lockup you describe as stock.

Its been a while since I've been an active subie-owner, so I'm not current on the newer systems, but if you have other supporting data I'd be happy to read it.

M
 
In the older AWD (all-wheel drive) autos, the Duty C disengages when the power is applied...there is actually a fuse holder under the hood that you place a fuse in to tow the AWDs, its even labeled "FWD". This is the system used in the older Outbacks, Impreza, and Legacys. You have power to all four wheels going down the road, the Duty C is powered when the car is placed into a turn to prevent binding, but they still bind some.

The lockup switch is used on the full-time 4WD (four wheel drive) cars, and those operate differently than the AWD. The full-time 4wd was used in the Subaru XT6 to name one application. They operate as you describe, where the solenoid is actuated when wheel slip is detected.

They are two different systems, and I owned an 89 XT6 before I owned this 97 OBW (among other cars). So I do know how both work and respond.
 
The FWD fuse isn't for towing though, it's for if you're running an odd size spare on the vehicle so you can avoid damaging the transfer clutch as Subaru wants all the tires to be within 1/4" circumference. The auto trans AWD soob cannot be towed with any wheels on the ground. (That's what Subaru says anyway; though you probably could tow it with the engine running and the auto trans in neutral with the FWD fuse in). The FWD fuse does nothing unless the engine is running anyway since otherwise the trans doesn't have line pressure to disengage the transfer clutch. Yes if the duty c fails closed you're stuck in AWD because then there is nothing to bleed line prsesure off the transfer clutch plates.

The lockup mod is pretty cool. Even when the TCU wants to put max power to the rear though it never completely turns off the duty c. All the solenoids it controls between 5% to 95% duty cycle at about 50 Hz.
 
deeter--

that's totally cool- I never knew that. I used to spend lots of time on the Ultimate Subaru BB and that info was even lost on that forum.... wow. But I like that approach more than the FWD with a part-time rear.

I had the 4wd Loyale which was quirky, but capable, and would have been a blast if they'd kept the 4-lo from the GL.

Also owned a 97 legacy, but it was a 5spd.... and it's AWD was absolutely fantastic, especially in the snow... 4-wheel drift rocket at 20mph.... good times... So-- while I did all of my own work, it was on a manual. Thanks for the writeup.

Much Respect,
Mike
 
Originally Posted By: Captain_Klink
apparently, the automotive engineers have been using ATF with hypoid gears, with no problems.
http://www.irday.com/html/Transmission Driveline Hybrid Drive engineering/20080412/7853.html

also, the Subaru engine is driving ALL four wheels, so the load on each differential is less that, say two wheel drive, which is actually one wheel drive.


ATF in manual transmissions ios very common. Going back to the 60's Dodge factory-filled with such in man trans, and today I believe just about all new cars with man trans use such. But ATFin a hypoid or spiral bevel differential is a tottally different matter and a bad idea. An ATF fluid does not have tha add pack to deal witthe very high pressures with hypoid or spiral bevel gear sets.
 
My Toyota's front differential uses ATF. And, as long as it is a synthetic, there is no wear on the magnet after 30k when I change it.

You can use ATF on a spiral bevel gearset. But, you are just limited to the amount of load you put on it. Nothing is impossible. I would never use an ATF where a gear oil is required, as the load, for that application, requires it.

Its foolish to mix ATF and gear oil. If you want to use a thinner gear oil, step down a weight. Pretty simple.
 
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