Just purchased some gear oil for the transmission. I am going to use AMSoil MCV synthetic motorcycle 20w-50 oil. Talked with their tech and he said it would be a great choice.
Motorcycle oil features better heat resistance, wear protection and rust preventative than motor oil. It is engineered for manual transmissions and meets SAE 90, API GL-1 non EP gear oil requirements. It also contains 1440 of ZDDP which attaches to the metal and is the secondary wear agent after the oil.
It is a GL-1, so there is no problem with brass syncros.
Mobil 1 also makes a comparable motorcycle oil that is NOT GL-1 rated. Called them and they said that they produce no GL-1 oils. Not that it couldnt pass, its just that they dont test for that spec. anymore. So I passed on it.
Regular GL-1 oil is just straight mineral oil.
I am just giving you options on these items. Oil changes on trans are so infrequent that a little extra cost doesnt matter to me. Rebuilds and replacement part are expensive and time consuming.