I use a quality 20/50 synthetic specifically sold for motorcycle use in my old Triumph and have noticed a general improvement in the way the engine runs. Haven't tried it in a Vincent yet but I don't see why it shouldn't be alright. A couple of Vincent riders here have been using it for three or four years and have noticed improvements as well. It seems particularly good in engines that see a lot of use in urban or semi-urban areas, with more than the usual number of stop-start short journeys. Of course, I have nothing against using traditional mineral oils, although I have rejected monograde oil in favour of multigrade oil (20/50) as the latter is far better than the early multigrades that really did play havoc with old motors. The classic motorcycle press tends in the main to be against any deviation from straight oil orthodoxy but that is because they wish to avoid upsetting advertisers and sponsors who produce and sell "classic oils".
PK