[Thor]

I can't see Geoff's comments on this so I may just be repeating stuff he has said.

My advice would be that modern synthetic gearbox oils should not give any trouble at all in old gearboxes as long as the actual fluid viscosities are about right. Synthetic oils can be thinner in viscosity, while still providing the excellent lubrication of an older oil which achieved the lubrication quality through being thicker.

The really dangerous thing you must be careful about is to only use EP type oils in gearboxes designed for EP oils. Syncro rings are sometimes made of bronze and as such are not compatible with EP oils. I am no expert on Bristol gearboxes so I can't tell you whether this is relevant.