Hi Chris, the correct oil for your trans if it is a sm420 4sp granny low trans is the GL1 mineral based oil. You also run this in your differential. Anything other is not needed or correct either. Some talk that your other GL5 oils are too slick to use effectively on the sychros to slow the cone down to make a smooth shift. Others will argue that GL5 will work. I personally assembled more than 1/2 dozen sm420s this last year and all got the GL1 oil and no complaints either. The trans will take 6.5 pints, if you have an enclosed ujoint make sure you get some oil in there too, there is a small 1/4" plug to remove on top of the rear bearing housing used for this purpose.

Cheers,
Steve (roots from Norfolk in East Anglia)

Last edited by sstock; 02/26/2019 1:59 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