Originally Posted by dabnis
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.
Forgot, I think the lead was also supposed to help lubricate the valve guides.