Hi Dick, I'm using 75W-90 oil in my 9" Ford rear axle. It's lighter
than 90, and I have to think it has less drag, especially since you
can't really get the axle hot before you run. I believe Valvoline
regular 75W-90 is perfectly adequate, and that the synthetic that costs
twice as much, doesn't really do anything more. On the other hand, some
racers swear by synthetic.
Many years ago I read in a Hot Rod article, "It's what you believe that
counts." How true that is.
Cheers from ArdunBill in the Great Dismal Swamp, temperature was 70
today, just right for motorcycling.
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