Here's a cut and paste from something I posted on another board a couple days ago just cuz I don't want to type it all again;

For some it might be useful to understand the mechanism of exhaust valve errosion infrequently experienced with unleaded fuel. Basically the valve face tries to weld to the seat face (at the microscopic level). Then the weld is torn by the valve opening. The notion of "lubrication" from the lead is somewhat of a misnomer from the normal way folks think of lubrication. Anyone who is familiar with welding knows it's difficult to weld dirty work pieces. That's pretty much what the lead did, it deposited on the exhaust valve area/components and prevented the welding by contamination. BTW, since the tetraethyl lead is in solution before combustion it does nothing for the intake valve. Using logic, the trigger for welding is heat. Vehicles that experience exhaust valve errosion usually are ones that work hard (i.e. not normal driving). Examples would be heavy duty equipment, racing applications, and so on where high heat is generated/retained. Another experience of welders is the difficulty of welding dissimilar metals, thus the stainless valve/induction hardened seat/ and so on.

I'll second Pat's comment about the dangers of a "less experienced" shop cutting into the water jacket under the seat. I had that happen on a BBC a few years back, took a while to show up so went undetected for some time.