Lubriplate makes the exact oil you are looking for, their SPO-288. I found it available in quarts at WBC Industrial in Colorado Springs and I've been using it for many years in my '36 Chevy pickup that I've had since 1965. With the Lubriplate product the '36 transmission shifts much better than it did with any parts store gear oil and better than with so called "600W" oil offered by so many vendors. Interestingly that "600W" stuff isn't even gear oil. It's steam cylinder oil.
'37, another good option today is one of the GL4-rated gearbox-specific oils. They are brass/bronze component and synchro-friendly, and have the right slipperiness (coefficient of friction) to promote smooth shifts, regardless if the transmission uses any "yellow metal" or not. GL5 gear oil designed for use in hypoid rear end assemblies is not recommended, as it is corrosive to brass/bronze (for transmissions that have that material) and may not have the correct coefficient of friction to promote smooth shifting.
I use Red Line Oil's MT90, since Red Line products are what I have in my shop for several newer vehicles I service. The Red Line products have been known to improve shifting in particularly balky vintage BMW transmissions, FWIW. There are other brand choices for GL-4 gear oils, so ignore the sales pitch, but here is a good white paper from Red Line discussing Manual Transmission Fluids [redlineoil.com]. Note, the MTL that is mentioned is really too low in viscosity for the older transmissions, but the MT90 is fine.
Thanks, I'll try that. This is my first project and I have a lot to learn. Apparently things have come a long way since plain old 90w. A while back I posted about the leak in the back of this tranny and was going to drill out what I thought was a frost plug. Was advised that it was a shaft not a frost plug. I got it cleaned well and sealed with Permatex and that solved the problem. That advice saved me from ruining the transmission.
The idea that GL-5, and to a lesser degree GL-4 oils are corrosive does not seem correct. That would mean they were effecting the yellow metals even when sitting in storage. The best information I could find about the effects of the additives in GL-4/5 oils is this ;
In normal operation, the sulfur/phosphorous additive forms a black sacrificial coating on the gears and anything it touches with a little pressure and temperature. As the gears turn, instead of wearing, the sacrificial coating of additives is peeled off or worn off. This is normal and acceptable in all steel gears. But when one or more of the surfaces is brass or another soft metal, the sacrificial coating is stronger than the base metal, and instead of just peeling off, it takes with it a few microns of brass that it is bound to.
I have no expertise in this area but it would seem to me that the only time these oils can have a negative effect is during shifting. Perhaps that is why Novak, a generally acknowledged expert in SM420 transmissions says GL-5 is fine in these transmissions. I will probably never put enough miles on my truck for this to become an issue but to use a phrase I have come to dislike, using "an abundance of caution" I am probably going to drain my GL-5 and replace it with something not known to cause problems.
Lubrication engineers have told me that the GL5 potential adverse effects on yellow metals can only occur at elevated temperatures (250-300* F) that do not exist in ordinary car and truck applications. So the question becomes whether to believe educated professional lubrication engineers or "anecdotal" opinions.