Originally Posted by BigRed03
"API viscosity ratings for gear oils are not directly comparable with those for motor oil, and they are thinner than the figures suggest. For example, many modern gearboxes use a 75W90 gear oil, which is actually of equivalent viscosity to a 10W40 motor oil."
My bike takes 10w-40 and i KNOW that its at least half as thin as 75W140 from dealing with both more times than i would care to count, same goes as merc 5 its water compareed to 10W40. Ford to my understanding does not run 75w90 in ANYTHING anymore, all the manuals are more of a "automatic tranny fluild" than a gear oil because of the tolerances being alot tighter than a 1967 ford f150, even the rear diffs are not 75W90 they are mostly 75W140