suzy q antiques,
I think the SM420 and a Muncie 420 are the same thing. The quandary over which gear lube to use has to do with the amount of sulphur (anti-wear) in the product. The brass synchronizers react with sulphur and are eroded.
If you use the incorrect lube, my guess would be that in about 99 years you might see a problem.
I believe Novak Transmissions, who deal with this box quite a bit recommend GL-5. This oil thing is Voodoo territory.
Stuart
