For those of us that have purchased Vincent motorcycles with unknown engine internals, and if you are anything like me, and prefer to leave a good running engine well alone, there is no way of knowing what material your exhaust valve seats are made from, other than waiting for problems to arise.
I therefore prefer to use Esso Synergy Supreme 99+ and then add to it genuine Tetraethyl Lead in the form of Tetra Boost E Guard 15, I would not dream of using any of the lead substitute products, lead has a proven track record over the last 100 years, you have to take care when mixing it with fuel, it can be a very dangerous substance, but the safety instructions are very well explained with the product, I am not breaking any rules, and I do not believe that the amount of lead coming out my exhaust pipe is going to significantly damage the planet.