If you haven’t had an unleaded conversion, then the FBHVC recommendation is an additive.
They endorse Millers VSP Plus, Redline lead substitute and Castrol Valvemaster/ Plus
Engines that are used constantly at the top end of their rev range are probably more likely to be vulnerable to recession than those used in a more leisurely fashion. For this reason I try to avoid long motorway thrashes.
Personally, I wouldn’t dangle fuel additive balls in my fuel tank, even if they were deemed good enough for Rolls Royce Merlin engines.
Editor FP