'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. Note, the MTL that is mentioned is really too low in viscosity for the older transmissions, but the MT90 is fine.
