[Mi_ka]

I just did an ATF refill in my my small Daihatsu Sirion (1.3 inline 4 / 100hp, 4spAutomatic with sportshift buttons, just 900kg unladen weight) and found out that the ATF is used to lubricate the front differential too on this vehicle.

Why is separate gear oil used to lubricate the front differential on the Forester and other Subarus? (..being also an Achilles' heel for the 4EAT if cracks develop because of possible TC overheating causing smallish leaks that can destroy the diff over time).
Why is not the diff just left to be lubed by the 4EAT's ATF?

As you guys over the pond have mostly automatic cars on your roads, what is the general rule for FWD transaxles over there? Separate gear oil for the differential or ATF?

 

[dpm]

depends whether the gears are helical-cut or not, at a guess.

 

[flstffxe]

The helical cut ring and pinion that generate a 90 degree directional change of power transfer create a pressure more then what atf is capable of standing up against.

There are plenty of manual transmissions that have helical cut gears and run aft as lube, but there are no differentials that do.

 

[Tremo]

ATF is totally unsuitable for use on a hypoid gearset. You need GL5 rated oil for hypoids with EP additives. ATF does not have the EP adds.

I wish they separated it on the 5MT like they do on the automatics. That way you could run a good GL5 for the hypoid and then a good GL4 for the synchronized tranny.

 

[Forester 2.5 XTi]

good point

 

[Woody '04]

The helical cut ring and pinion that generate a 90 degree directional change of power transfer create a pressure more then what atf is capable of standing up against.

There are plenty of manual transmissions that have helical cut gears and run aft as lube, but there are no differentials that do.

+1, this is the key.

BTW the Chevy Suburban I had ran ATF in the transfer case, which was a surprise to me b/c smaller Toyota had gear oil. But again, no diff in the t-case, only gears.

 

[Tremo]

+1, this is the key.

BTW the Chevy Suburban I had ran ATF in the transfer case, which was a surprise to me b/c smaller Toyota had gear oil. But again, no diff in the t-case, only gears.

ATF works good enough as a low viscosity gear oil. (Not hypoids.)

Tremec manual transmissions call for ATF. The T5 in my old IROC Camaro runs a 50/50 mix of M1 ATF and RP Synchromax. Works fine.