[Excaliber]

Well that was an interesting reference Ed, good post with a nice balance between the myth and the facts.

Sounds like you would have to run the engine for some time before a valve burned, I'm not sure how long that would take. It reminds me of running unleaded fuel and burning a valve designed to run with leaded fuel. Yes, you could burn a valve without the lead IF you ran down the highway at sustained cruising speed for many many miles. Perhaps several hours.

I suspect a larger concern, and this may be an oft repeated myth, is cold air entering the exhaust chamber and warping the hot valve immediately after shut down?