I've heard good things about Red Line lubricants, their 'Superlight Shockproof' gear oil has been recommended as being the most suitable Red Line type for Triumph 5 speed gearboxes, so I don't know if it would be more suitable for the Norton box than the 'Heavy' or 'Light' types? It isn't generally available in the UK so I've not tried it yet.
I'm using Red Line MT90 in my Norton's gearbox. Works great and has a GL-4 rating, which I'm told is important for the gearbox bushings. Hard to find lubricants that are not GL-5 these days. Info from their website:
I run the RedLine Light Weight in a Jerico race transmission on a car and the Super Light weight in the gear box of a Honda Moto engine on a shifter kart. Both are excellent. In addition, if you have a leak the color confirms the source.
Was perusing the records at the factory today and found a catalogue/designer hand book for Bronze bushes, it stated that you only needed to worry about the oil degrading the bush if the oil operated over 100 Centigrade. So we can all stop worrying about GL-4/GL-5 as the real consern is the oil never getting hot enough to get rid of mositure.