Maybe it is the fuel used? I always run good gas, the little extra in using good fuel saves in mileage and repairs in the long run. Tune ups last longer and less sludge in the engine.

Whether the tank truck says Shell, Chevron, Mobil, or Beacon, 7-11, or Mom and Pop, they all fill from the exact same spigot at the refinery.  Doesn't mean there aren't differences, because there are...in the additive package, not in the base fuel.  The additive package is put in to spec when the truck is filled.

Unleaded fuel dates to the mid - late '70's, not the fifties.  Any domestic gasoline before this change had pretty high levels of tetraethyl lead in it, which also drove the high octane ratings of gasoline from that era.  Any domestic vehicles made before this had unhardened exhaust valve seats.  As the lead in fuel promoted cooling of these seats, it's not uncommon for these older engines to burn exhaust valve seats when using unleaded.  The cure was switching to a hardened seat, typically done with a Stellite coating on the seat/valve, which better resists the high temps.