Kevin,
I have played around with many different gear oils and the best for shifting is the NAPA 90weight mineral oil. They sell it by the gallon, you will need a tad over 6 pints (granny low sm420).
I've had a dozen or so sm420 opened up on the work bench this last year, playing with all the synchros on the gear cones and the mineral oil grabs these synchos the best, the worst was Redline 75-90 synthetic, it was too slippery. About 6 others just as bad as redline all synthetics too.
Looking at the Napa Site, I did not see any mention of Mineral Oil. I see this one available online in 5Gal in Quarts. "NAPA Premium Preformance 80W85W90W Gear Oil - 1 qt", NOL 75210
Is this the right stuff? if not, do you have the part number?
