I am not sure why you would want to add a lead additive to a gas engine. I do not have a gas engine on a boat but was under the impression that you do not want lead in a gas engine.
IIRC, the lead acted as a "cushion" for the valve and seat contact. Older engines may suffer valve related problems without an addetive. I had an older Continental flat head engine and had to go with stellite valve seat inserts and stainless valves, or maybe the reverse? It was a long time ago.