i read most people like to use either a 75-90 oil or a 75-140 in the tranny. my work uses the royal purple 220 weight. it is a food grade synthetic oil and has been filtered to the nth degree. I can thin it down with some 30 wght if need be. We use it in our high speed bearings and all our gear sets. it is clear and even when we drain it out after about 90 days or longer you can still see thru it. the machines all run 24/7 365 in temps exceeding 265*c. Dont want to use if it might be too thick. it actually flows easier than a 90 wght oil maybe becasue it is synthetic.