So neither the Comma 20w5o nor the Duckhams Q 20w50 are recommended for our Nortons! That's a shame, for example in the UK the Bel Ray comes out at about three times the price of the Comma, to which I think the Duckhams is priced similarly, and I'm always on a tight budget. I'm left wondering if those two mineral oils are even any good for my old Triumph Herald car, which has a similar valve train to our Nortons but is of course is water cooled … Any thoughts, Jim? If not, thanks for the testing, anyhow!
In the meantime, the Castrol Power 1 4t 20w50 looks like a good option for my Commando at its UK price, from Opie Oils on eBay, less than half that of the Bel Ray. Although my Commando is at standard tune.