When I had my shop, many years ago, We used Kendall Black Label 20 50 or DA Speed/Sport in the Nortons and BMWs and Torco 2cycle in the Bridgestone and Bardahl Synthetic in the Montesa's. But then we were dropping the oil every 1000 to 1500 miles in 4 strokes and the 2 strokes were self changing
Yes, I used to change the oil every 2,500 miles on both my BMW R100 and a Laverda triple years and years ago. My experience of my friend's Norton Commando was it seemed to drop many more things than just oil every 1,500 miles (Lol).
I think the oil I used back then was ELF as it was the one recommended oil brand listed for both bikes. The only specialist American oil I can recall in the UK back then was Bel-Ray.
The BMW R100 did over 100,000 miles before I gave it to a friend when I emigrated to the US. It still ran well enough for him to tour Italy on it from the UK and well beyond.
My Yamaha RD350LC I actually ran factory Yamalube in it, and that ran about 40,000 miles before a much needed full top and bottom engine rebuild. That was pretty good mileage for one of those though.
I think you can't beat regular oil changes with a quality oil and filter of the correct specification. Which brand of high quality oil doesn't matter as much in my mind as making sure it (a) meets the manufacturers specification and (b) it's changed regularly. Everyone has their own favorite for sure.
Never in all my nearly 50 years of riding and driving have I had a directly oil related failure. Even my children's beater cars when they were younger, I used to do oil changes every 5,000 miles with synthetic oil at Jiffy Lube and they still ran to high mileages. I run Jaguar Land Rover vehicles and have never had the problems that other people have had when they follow the JLR insane recommended 10,000 to 15,000 mile oil change intervals. I've done mine every 5,000 miles and had no issues even with their recommended "water like" 0w20.
Oh happy days.