Molybdenum disulphide is a common additive to gearbox oils.
Many motorcycle manufacturers recommend them for the gearbox/final drive as an additive to the gear oil. Moto Guzzi, and BMW being a couple of the above. (these motorcycles have separate gearbox/final drive oils, as opposed to the japanese motorcycles, which use motoroil for everything).
I have personally used MoS2 in Amsoil gear oil (in motorcycles), without any problems. The key to remember is to add the right amount (which is very small).
Havins said that, here's a site that shows a study which claims that these lubricant additives dont work:
I dont see a harm in it, but I havent added any to my gearboxes on my D90 (yet).
Actually..I never thought about it, even though I have a bottle sitting in the garage...
Maybe I'll give it a shot.