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Friction modifier and rear diff fluid?


NAXJA Forum User
NB Canada
Hi. I am about to change my Rear Diff fluid. My rearend is the Corporate 8.25 with a trac - lock limited slip. My Question is if I change to synthetic fluid that states it is for limited slip applications do I still need to add friction modifier or is it already in the fluid? Will putting synthetic fluid in with residual regular fluid harm anything? What is the best way to clean out the old fluid? I was thinking Mobil 1 synthetic fluid. Is this good or should I get Mopar? What about friction modifier. Is Jeep the only place I can get this stuff? Should I just buy mopar original fluids (non-synthetic) with friction modifier made by Mopar to keep things simple?
I'm no expert but from what I've heard & read for a diff with anti spin especially one that does alot of towing should use 75-140 syn & modifier. Most guys I know use brake clean to clean out the old fluid after removing the cover. Ask your local jeep dealer parts &/or service people for more info. They usually are happy to help if ya catch them in a good mood that is, lol.
There are some synthetics that claim to have an f/m in them already. Still, I'd use an f/m from the Big3.
The local shop I deal with has an AC-DELCO friction modifier they use in axles. IIRC $10 for the bottle.
They also use it in Transfer cases that have the clutch packs,IF the owner complains about problems in Full-time 4wd.