In terms of viscosity, OE called for 90 weight, now normally 80W-90. 75 or 85W-140 provides more high temp protection and more "cushion" at lower temps and likley causes a bit more friction and HP loss. Some racers have experimented with lighter lubes for slight ET improvments... I don't have a spare to throw in if mine breaks in an "experiment" so I'm running 85W-140 for the better protection as recommended by the builder of the rear. I've also used Autozone limited slip additive with good results (8.75 clutch type sure grip).