Which would be closest to 80w-90. 75W90 or 75W140 or 85W140?

 

id say 75w140,
Tip;
Redline 75w-140NS for the transmission (excellent for old synchros)
Redline 80w-140 for the rear differentials

hmm what are you using the oil for?

 

i want to change to royal purple synthetic gear oil but they dont sell 80w90 in just quarts, you have to buy it by the case.

 

is royal purple just as good as redline?

 

Yup royal purple is just as good. It's just that alot of 240sx owners i know use redline.

 

I put in Royal Purple 75w-90 in my tranny and in my diff about a month or so ago. I called royal purple, and the guy said the only difference is how the oil will act when it gets below a cetain temp, and he said it won't really make much of a difference at all. I really like the Royal Purple, it made the drivetrain seem a bit smoother. I would highly recommend it.