Just tested it
______________________________________________________________
Valvoline_Motorcycle_20W50
201 lbs load
4.09 total heat from friction
3.578 heat from viscous friction
2.71 heat from High pressure shear
Safe for a wet clutch
OK for a Norton in moderate climates, too much heat from friction to use hard when it's hot.
________________________________________________________________
Kendall_GT1_High_Performance_20W50
!40 lbs load
4.73 total heat from friction
3.636 heat from viscous friction
No data for heat from High pressure shear
Not recommended for wet clutch
Not suitable for a Norton
And now for the big surprise. This is the oil I use in my 68 Mercedes so I had part of a bottle laying around. I did not expect it to do this well.
__________________________________________________
Royal_Purple_HPS_10W40
458 lbs load
1.78 total heat from friction
1.026 heat from viscous friction
1.7 heat from high pressure shear
Not safe for wet clutch
Great result and certainly safe for a Norton but it may cause excess oil consumption in hot weather due to it's low viscosity.
I have some 20W50 HPS on the way for testing. If the 20W50 tests that good with it's very low heat from friction and more than adequate load capacity, it could be the best bet yet.