Lead (TEL) was added to gas in the early 20's (Hello, Ethyl!) and was phased out beginning in the 70's.
The lead was the "lubricant" that kept the deposits off the valve sealing surface. When lead was phased out, valve life took a bit hit, thus the need for hardened seats and induction hardening.
Some engines, if used lightly and little, may last a while, but even highway driving can ruin an engine in a relatively short time if unleaded fuel is used with unhardened seats.
You make your choice and take your chances.
