I'm replacing the rear differential fluid on my '03 Legacy GT with 75W90 synthetic. It has limited slip rear diff, do I have to add any friction modifier additive for the LSD? Any advice would be appreciated! Brian
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...