I had some Redline MT-90 lying around and need to change the differential fluid. I'm probably going to use RP 75w-90 but just curious what would be the side effects of using the Redline.

 

AFAIK, the MT-90 is only for Manual Transmissions, and it doesn't have the LSD additives that your differential needs. Redline makes a GF-5 gear oil ("75w90 Gear Oil"), specifically for limited slip differentials. Shouldn't be any issues with that or RP Max Gear 75w90.

 

the moment you put MT-90 into your Rear Diff, its the moment your Diff getting its slow and painful death.