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.

Last edited by sstock; 09/30/2019 7:10 PM.
1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
In the DITY Gallery
Video of the 261 running

1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration