Jason S,I believe the tin is supposed to serve the same purpose as the lead in fuel did.It aids in conducting heat from the nearly red hot exhaust valve to the valve seat and heat.This happens when the valve is on it's seat,a very short length of time.The seat erosion comes from micro welding when the hot valve hits the seat,then the valve opens taking a little seat material with it.Over time,the seat is eroded away.
Generally,the harder the engine is run,more seat erosion occurs.This varies depending on engine design,some are worse than others.
I've worked on a few Chevy 235-261 and GMC 302's every one of these heads showed signs of erosion and the exhaust seats where pounded down from wear.
A machinist can "find" a seat when grinding the valves no problem,but the valve head is recessed in which will restrict airflow and have the valve stem standing to high,requiring a pile of spring shims to get the correct spring installed height.
