I have always used the Hypoyd gear box oil all my Norton's life, I still have very light gear changes and a light click to netrual, gear box was rebuild back in 1979 from the dreaded lay shaft bearing and replaced the kickstart pawl 4 times and the only real wear on one gear was the kickstart gear where the pawl locks in for kicking, it was replace a very long time ago all bushes were replaced in 79 and are still inside, on my lay shaft the bearing was replaced with a tapper roller bearing back in 79 this bearing at the time was called the quick change bearing that the racers used in the days for quick changing gear ratios, this bearing is still inside my box, I have never had any oil leak from the kickstart shaft and shafts have never been shimed, so I think I will stick to my Hypoyd gear box oil, its been working well for me and all oil level have been messured by the reconmended level in my workshop manual.
In my younger youth days my gear box copped a hiding from doing burn outs to very hard reving between gear changes, my lay shaft bearing blew right on 10,000 miles and blew when I put it into gear when leaving home and the new quick change bearing was given to me all though years ago.
My gearbox has well over 160,000 miles on it and its still changes sweetly from the first day I brought my Norton new, so the Hypoyd GB oil must be doing its job, but I wonder if the tranny oil would have done the same but I am not going to find out I stick with what works for me.
Ashley