I think it is time to reassess the oil you are thinking of using . The spec/additives for oil over 80 years ago was quite different to todays product .
My experience has been 80,000 miles of daily use , original 1940 , 4spd (non-syncro) with new bearings in it 22 years ago.
I was advised by a friend who was workshop manager for a road construction company that they used a Castrol GP50 engine oil (rated at 90 gear rating) for their old trucks ,
I have found the cold running gear changes easy and smooth , hot running just the same , I often carry the trucks full load (camper) and the g/box is still as quiet as you would like .
(just look at those beautiful 80 year old gears