and as for monkey pant comment on diesels can you say audi diesel one of there cars has won over two hundred races , its almost reach the status of slippery sam
So the general consensus seems to be 'please yourself'!
I've used everything from Sainsbury's multigrade - no longer available I believe, but recommended by the late Ian Hamilton - to whatever has been available from my local Halfords (usually Castrol of some type). I've never used a monograde oil in the Vincent but do change whatever I am using often. I change the filter after every other oil change. This of course is a complete anathema to the modern vehicle owner whose car (in the case of the Cadillac) requires its first oil change at 100,000...or so I heard!
All I can say as a complete non-techy with extremely limited technical ability, is that it makes sense to me to keep our now-elderley engines refreshed with the cleanest, newest oil. Come to think of it, that's a suitable analogy for ourselves - make mine a pint of Harveys.