I had read around the subject previously, and have come across a few articles describing the relation of lead to valve seats as another "conspiracy theory" propagated by oil companies for various reasons.
Unfortunately it is all unsubstantiated with no hard scientific evidence, hence this posting.
Presumably one way to test it is to run unleaded until something happens. However, I was hoping someone has already done that and have an answer. Always good to learn from someone else's mistake...............!!!