I switched to this oil as well, since it performed near the top and is available in the US on Amazon and Ebay at a reasonable cost.
Note, though, that Royal Purple makes several types of oil. Not all tested well, including some sold at higher prices than the HPS ("High Performance Street") variety.
Given the high wear that seems to afflict Vincent cams and followers, I think oil film strength is critical.
Re the Royal Purple, that is correct , the really expensive full synthetic that Jim Comstock had been using, tested quite poorly.
He was using the really expensive stuff to protect the high lift cams in his hotrod Commando engine.
When the cams and follower wore out at 25,000 miles, he started in with the oil testing. He learned that his oil choice was likely the culprit in the early cam failure.
Norton owners from all over the world sent their favourite oil to him for testing. The results were pretty interesting.
You will often hear people claim that all oil is the same as long as it has the appropriate API designation. This is not so.
One of the scariest results came from the Aeroshell oil.
This is a high Zddp product marketed for use in small aircraft engines, Cessnas and similar.
The oil turned to smoke as the first small increment of weight was added to the scar tester arm.