straight SAE20 every time. I have the triumph 4 spring in a BSA B31, M21 and Triton. No problems with slipping with SAE20.
Modern gear oils are full of friction modifiers which I can't see working with this clutch. I would suggest you strip the cutch, clean the plates with petrol to remove any oil and rebuild. If you grease the bearings, primary chain and the engine shock absorber well, you can run it for a few miles to be sure the clutch is set up correctly before adding oil.