The best answer is from experience, I know of some who have run full synthetic in their Vincent, But why go to that expense......It's not like the parts are unavailable, and as many on this forum know, most bikes don't get enough use to actually ware them out. There is a very real possibility of the rollers skidding across the big end pin, I'm not sure I would take the risk. It is not worth the extra expense for synthetic in such an old designed engine, and there are too many people who take advice from expensive magazines who actively promote new and expensive products that are not necessary on these old machines. If you seriously think that running full synthetic in your Vincent will keep it going forever, well then use it........Like I said, you can't beat regular oil and filter changes, and don't use a heavy grade of oil.