"Transmissions with brass synchronizer components are generally not suitable for use with GL-5 specification oil unless specifically stated by the manufacturer as the extreme pressure (EP) additives in the oil are corrosive to brass and bronze components at high temperatures and decrease the synchronizer effectiveness at low temperatures. The additives in GL-5 oil also cause physical damage to brass synchronizers as the EP additives bond more strongly to the brass than the brass does to itself, causing a small layer of brass to be worn off with every gear change.[6][7] Instead, oil which meets only the GL-4 specification should be used whenever possible."
https://en.wikipedia.org/wiki/Manual_tr ... ynchromesh
