Viscosities mentioned in this post are pretty far apart ?? 0-20 & 75 ? From my 60 some yrs experience w/ transmissions , 75 seems more applicable but then todays lubricants are nothing like yesterdays for basically the same mechanical mechanism --- but---- w/ todays ability to hold tighter machining tolerances along w/ very high bearing quality leads me to suspect a much finer , closer tolerance mechanism not requiring the heavier viscosities of days gone by .