always thought it was weird 75-90 feels much thinner pouring than 80-90. I know one is usually synthetic and the other not but it's a noticeable difference. doesn't surprise me 75 90 is as thin as 20w50.
red line MTL is called a 75-80 gear oil but I sware it pours like a 5w30 or thinner motor oil. corvette guys say mtl is to thick for their manual trannys that call for dex 6. so they mix 2qt mtl and 2 qt atf. I honestly can't tell a difference, always run straight MTL 75-90 in trannys that call got dex 3 and 6 with good results and sweet wot shifts.
some older manual trannys called for straight sae 30w motor oil.