if you like rp.....feel free to use it, it has friction modifiers,but they wont hurt a thing! alot of guys(and gals) are using 75w90, and its fine for a rear diff( lsd or locker). im personally switching to mobil1 75w90, more than likely, here in a couple of weeks. different people will say this is better or that is better, but it really depends on what you can afford or what you like. mobil1, royal purple, amsoil( all synthetic mostly) are very good!!I Have Searched And I Can't Really Find Any Thing About What Type Of Fluid To Use. I Like Royal Purple 75-90 But It Has Additives For Lsd, Is It Ok To Use, Has Anyone Else Used Royal Purple? Or Something Better?? Thank In Advance For Your Help..