Pan Head bronze valve seats...

[T. Cotten]

....so will they run ok with the unleaded gas?

Is there lead in propane, Steve?

Tetra-ethyl lead was added to gasoline for one reason: Octane.

The early 1980s 'unleaded kit' fad was at least 97% sales pitch.
Early Shovels had hard seats. Then the MOCO went soft, and certain valves would slowly micro-spot-weld to the seats with the elevated temps of "Energy Crisis" lower octane fuels.

Lead was claimed to insulate against this, when it's major benefit was just to lower the combustion temperature.
Beyond the stinky politiks, ethanol is added today for octane.
(95%ETOH/5%H20 has 95 octane, according to Mike Brown's Cookbook.)

....Cotten