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After a bunch of research and many inquiries for information here is the list of oils I would suggest for use in a Norton motor.
There may be others that would be suitable but note that if the oil has a GF-4 or GF-5 designation then it is NOT suitable for an air cooled, flat tappet engine.
Here is my list for oils that would be suitable
in a Norton motor. All are capable of high temperature
operation and have support for flat lifter cams.
ZN PHOS
Brad Penn Grade 1 20W-50 #7119 1540 1319
Mobil 1 20W-50 (V-Twin) 1600 1700
Mobil 1 0W-50 Racing 1750 1850
Joe Gibbs conv. 15W-50 high
Joe Gibbs synth.15W-50 high
Amzoil Dominator Racing 15W-50 high
Royal Purple XPR 20W-50 1200
Valvoline NSL Synthetic 20W-50 1400 1300
I did not include Valvoline VR1 racing oil because
it has changed formula in the past couple years
and is now rated for use in emission controlled vehicles.
I have not been able to get up to date data. The request
to Ashland oil for information was not answered.. Jim