I totally hear you. I'm in the same boat as you kind of. Lol. I pulled the motor with the intention of installing the 4 speed so I was all in. But the gear oil thing, I hear you. I get going down rabbit holes also. I think I'm just going to go with 75-85 gear oil, non synthetic. I know AJ uses a 50/50 of dex III and gear oil. I'm not sure where you find dex III, it's all been superceded to dex VI which is syn.
I bought a brand new 73 Dart Sport 318 4-speed car back in 73 and the gears just rattled something awful. I found out that the gearbox came equipped with ATF so we drained it and refilled it with 90 weight and all the noises went away.
