AFAIK your rear LSD is a sealed viscous unit that does not come in
contact with the gear oil, and therefore does not require friction
modifier as does a wet-clutch type LSD. However you can still use the
additive as it will provide some benefit for the gears and bearings.
Some synthetics, such as Redline 75W90, already contain the modifier.
When in doubt, talk to the Subaru guys at the dealer's service dept.
Maybe call more than one dealer to make sure you get the same answer
more than once? BTW, what's the owner's manual say?
Then, look at the label on a bottle of your chosen fluid: I have both
synthetic and dino oils on the shelf in the garage right now. The
synthetic says "for conventional differentials and topping off LSD
types" (in not so few words.) The dino says "for drain and refill of
conventional and LSD types" (again in not so few words.) From that, I'd
gather this particular synthetic would require the addition of an
additive for a refill.
If yours does require an additive, call Subaru again and talk to the
parts guys to see what they recommend. Many mfrs have their own and it's
hard to tell which ones are just rebadged commercial products, which
ones actually proprietary compounds. The situations where a proprietary
fluid is required are increasing in number...
The manual says to use GL5 80W90 fluid and doesn't mention anything about
friction modifier. I'll call my dealer and see what they say. I just
figured there would be someone on this NG that would have the info at hand
rather than me call them and get the run around.
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