The lead compound used in leaded gasoline does not act like a lubricant. It acts like a contaminant.The lead in leaded gasoline acts like a lubricant, therefore engines running leaded gasoline could use valve seats ground directly into the cast iron head with no problems.
Years ago, NASCAR ran titanium valves with copper/beryllium seats. All was well until they tested unleaded race gas.
The valve/seat interface lasted about 100 miles before massive valve recession and seat extrusion occurred.
The problem was traced to seat contact pressure, valve rotation at close, higher temperatures and clean metal surfaces.
Under those conditions, the valves would micro weld to the seats and then rip out metal when the valves opened.
Tired engine that were blowing oil did not have that problem, nor did leaded fuels.
At operating temperatures, TML or TEL, will decompose into an oxide and phosphate which act like a contaminant to prevent micro welding. Hard to weld dirty metal. Since leaded fuels were going to be banned, one idea was to add lead to the oil and run rings loose. This worked but cost too much power.
The final solution was to use coated titanium valves that brought their own non-weldable metal surfaces.
Leaded fuels do not provide lubricity for valve guides because they operate at much lower temperatures.
Unleaded fuels necessitated the use of induction hardened seats or proper seats installed into the heads as a production process.
Yes, strangely aluminum heads with stellite seats never had the microweld problem. Now we know why.