I ran Royal Purple 75-140 full synthetic gear oil in my rebuilt 03 Cobra differential since day one. The clutch packs did not last more than 5,000 miles before they burned up enough to easily turn the wheels in opposite directions by hand when the car is jacked up. I did not add friction modifier as the bottle explicitly stated that it was designed for limited slip diffs and no additional additives were needed. Everyone says adding friction modifier isn't necessary, but is this really true? Has anyone added the Ford friction modifier to the Royal Purple oil?