I think it was Slick 50, just been looking it up on the net.
I don't think that Slick 50 is considered a legitimate oil additive by those who have conducted scar wear tests. I would put it with zinc and ZDDP as oil additives that do not work well. Unfortunately, oil with high zinc levels do not work well either. While it does work as an anti wear additive, GM found that it becomes corrosive at high levels. They have spent a lot of money trying to wean Corvette owners off ZDDP and they have changed their owner's manuals to promote low ZDDP oil.
There are oils that provide stunningly high wear protection right out of the bottle. Many owners are using these, which is not surprising. Even though there are additives that do increase wear protection, I would just use an "out of the bottle" oil that gives the same results. Glen mentioned many of these oils.
If you have had good luck with your oil, keep using it. If you are trying to find a new oil I would stick to the 40w oils (including multi grades). High viscosity oils give you a little reserve during high heat moments. Although Vincents rarely get their oil up to cold temps, I think the oil is exposed to some very high temps in the cylinder and cam spindles due to combustion and hydraulic pressure.
Oil provides the highest protection at boiling temps, but the Vincents do not get their oil tanks this hot. So, it is good to start out with a very good oil that performs well cold. Like everything else, oil is a compromise also.
David