One of the "everyone knows that" areas of "common knowledge' (which means "usually wrong") is that the lead was the lubricant for the exhaust valve. (oil would be burned off due to the temps of the valve) so only lead would do the job to provide a "babbit bearing" sorta lube for the valve stem and for the actual valve as it rises out of and sets back into the seat.
Tetraethyl lead was a main component in raising the octane of gas from the abysmal levels of pre-WWII to the stuff that hi-po engines could actually use.
When lead was abolished from gas in the 70's, many engines experienced valve recession and some also had greatly increased wear on valve stem guides and so the "common knowledge" came about.
I assume this is a Lycoming IO-360? (The one I owned for a while in a PA-28 was the 150 hp version and only needed 80/87 fuel) As I recall, the 100LL usually lead-fouled the plugs something fierce, but was a compromise fuel supposedly better than the (then common) 100/140