Out here in Australia we typically use monograde low detergent oils in vintage engines. 50 weight is most common, but it's generally hotter out here than in the UK so 40 might be better for your application. I wouldn't use a high detergent oil unless the motor is freshly rebuilt. Just a comment: Long ago I had a BSA Sloper that carried its oil in a reservoir in the crankcase casting. The oil got reasonably hot in there, so if your bike happens to be a sloper you might look at a heavier grade. If the gearbox has oil seals in the bearings, any heavyweight gear oil should be OK. I think BSA gearboxes have a level plug, so don't overfill or it will come out everywhere!
Leon
