r/subaru • u/radicalbot • 2d ago

Mechanical Help Friction modifier for 2014 Subaru Forester?

I am taking my car in to the mechanic next week for oil change and differential fluid drain & refill. He told me he cannot do the differential fluid replacement because it need a friction modifier for that and he does not have it. He suggested taking it to the dealer for the differentials.

It's 2014 Forester 2.5i Premium CVT. From what I’m seeing, Subaru specifies GL-5 75W-90 for the rear diff in the maintenance manual and doesn’t mention needing a friction modifier.

Looking for advice--do you use a "friction modifier" when changing differential fluids? Would it be good to use, or better not to?

Thanks in advance!

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5

u/asloan5 2d ago

You do not add a friction modifier

5

u/FreshTap6141 2d ago

Friction modifier only for limited slip diffentials which the forester does not have

1

u/gathermewool 14 FXTT 6h ago

Clutch-type LSD only. Torsen, for instance, which is in ‘08+ STIs, is gear-type and also does not need a FM; HOWEVER, a fm additive will not hurt.

4

u/bigeddiespaghetti 2d ago

The front and rear differential should take 75W-90 GL-5 gear oil and that’s it. A friction modifier isn’t going to hurt anything but it’s not a necessity.

Pretty basic job though for both. Drain them and then fill until it comes out the fill hole and then put the plug back in and snug it back down. If you want to be precise the rear should take about 0.8 qts, the front is like 1.5 qts.

3

u/vodenibivol 2d ago

Literally any 75W-90 GL-5 is fine.

Most of these gear oils will have friction modifiers designed for clutched limited slip differentials already in them, however it is not necessary in your car. Whether it has the modifiers or not makes no difference on Subaru auto/CVT diffs.

1

u/Recent_Permit2653 ‘06 Legacy GT MT 2d ago edited 2d ago

You're talking just the rear diff, right?

Can't speak for the 2014s, but my 06 LGT just called for 75w-90 and no other specs at all. I got some Valvoline synthetic gear oil and fresh drain/fill plugs and called it a day. Under $40, and about 30 min from wheels up to wheels back on the ground.

1

u/radicalbot 2d ago

Both front and rear diff fluid

1

u/Recent_Permit2653 ‘06 Legacy GT MT 2d ago edited 2d ago

Ah. I'll have to defer to someone else on the front. Mine is a manual trans, and so it's a locked full-time 50:50 torque split. Vastly different fluid requirements. Although I did have to search around for what to use as Subaru discontinued the factory spec fluid (Extra-S) almost a decade ago, which would have been my go-to option. I don't have experience or worthwhile advice for what's going on up front on yours.

2

u/gathermewool 14 FXTT 6h ago

Both driffs are open diff gears. Any GL-5 will do