Originally Posted by JFUSION

There isn't any friction compatibility issues, but I will swear that a thicker fluid provides different lock-up properties on my differential, I believe one other forum member agreed with my findings having experienced the same thing on his s2k.

When it comes to long sweeping corners under acceleration I believe the diff was locking up and causing understeer, and if I lifted it would resort to oversteer. It was worst with 75w90, it got better with 75w110, and I think it is perfect with 75w-140 (all Amsoil severe gear fluids). I hope to have a UOA sample of my 75w140 from this season in the next while. If the UOA looks good I'll keep running it into the future.