I found some gear oil in my garage that had the mil spec not the API spec on the bottle. The 1968 Pontiac service manual says, to use SAE-80 or SAE-90 multi purpose gear lubricant MIL-L-2105B in the manual transmission. Mil spec MIL-L-2105B Is the same spec as GL-4 lubricant, MIL-L-2105C,2105D,2105E is the same spec as GL-5. Thought this may help if you find some gear oil with the MIL spec only.

Also read a transaxle paper by a Corvair guy, he states GL-4 has only 1/2 the sulfur/phosphorous additive that the GL-5 has. That results in less bonding, and less extreme pressure protection, so the additive peels off parts easier and as a result takes less brass off the surface. In used oil analysis they found 2-4 times as much copper in transmissions using GL-5 as they did in those using GL-4.

The manual doesn't say anything about using synchromesh additive in the transmission, I never put any type of additive in motor oil, but I did put a bottle of GM synchromesh additive in my Muncie when I change the lubricant. Seemed to shift better, but that could have been the oil change, or maybe me self justifying spending the money on the additive.