Early gearboxes have parts that might be compromised by detergents, so I use 30 wt. non-detergent. Center shift 'boxes appear to allow multi-grade detergent oils (factory handbook) as used in the engine, so I use 20w50 GTX in the BJ8. No need to worry about ZDDP here. This is what I have concluded after years of debate and questions, and it seems to be working.
Bob