rear end fluid change
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Brands like Royal Purple and Redline already have the friction modifier so all you do is add the lube from the bottle and you're good, do not add anything else.
Using too much friction modifier is not good, I can't remember if it lowers the oil's viscosity or causes shear or whatnot, but you want to use as little as possible. And the only time you need the additive is when you have a clutchpack type differential such as the Auburn that I think was standard in 1998 models and optional from 1999 on.
With the torsen differential, it's all gears so you do not "need" the additive. the only catch is the differential may be a little noisy in slow tight turns and that is normal and does not hurt anything- you can get this info directly from zexel-torsen. On mine with the exhaust and normal drivetrain noise I don't think I'd never know if the torsen is making any noise, I just run 75w-140 motorcraft gear lube (which i got free from a friend) with no problems and that has no additive in it.
and friction modifier, limited-slip additive, is all the same thing.






