Ditto the 20W50, except I use Castrol in the engine and primary and 75W90 Royal Purple synthetic in the gearbox. I change the engine and primary oil every 1,500 miles and the gearbox oil every 3,000 miles.
When I restored my Commando I spent quite a bit of time 'chatting' on the phone with Les emery who has/ had HUGE input with the Norton and at that time The Norvil Shop in the UK.
He explained they had spent a lot of time testing and working with the (Australain) Penrite company develping their oils. He felt that the Pentrite engine oils are the best suited for the Comando engines.
This fact is endorsed by many many Norton fans around the world.
I could tell you if this product/s are available int eh USA though.