Hmm, say I should only use Amsoil 75W140 for my final drive? Disregarding BMW's probably vast knowledge and testing of proper fluids? Disregarding the 175,000 miles I have on my original, untouched rear final drive on my 94 RS, all run with synthetic 75W90 gear lube, and no problems? Thanks just the same but I know what works for my bike.