The past year has seen some very lengthy threads on the right gearbox oil to use in our 308, 328, and other F-cars' transmissions (gearboxes and differentials). There was a fair consensus forming around using Redline 75-90 NS, in some cases with a little differential "slip" additive added to prevent differential "chatter." In the past months I've read a number of posts referring to Royal Purple. What is this stuff? Anybody use it? Is there any research that compares the properties of Royal Purple (RP) vs. Redline?