30 weight oil is the alternate fluid of recommnedation for M46 with type OD after 1979/80 model year. It is allowed only if the temps never are below 10 degrees Celcius. Other than that Type F ATF is the fluid of recommnedation by the facory.

Without formal training, or actual data, the viscosity of ATF, 30 weight oil and some gear lubes are all very similar. Where the differeneces are is in the additives. Gear lube has a much higher sulfer content that the others and will provide wear protection by resisting being squeezed out from between gear faces as they mesh. All the others function much the same but with different metal in the gears can tolerate different contact pressures under bare metal contact.

I've never had a tranny apart with either lube. My guess is with proper levels and a fluid change now and then, most tranny's should not wear appreciably for a long time.

Duane