Old gasoline engines don’t need lead or lead substitutes to keep them healthy. That’s “urban legend” stuff.
In 1975 when new cars had catalytic converters and required unleaded fuel, the engines started being built with hardened valves. The hardening was needed to compensate for the “lubricant/cushion” effect that the lead once provided.
I remember doing valve jobs on pre-1975 cars (without hardened valves) that used unleaded gas over a period of time. A common problem was a recession in the valve-seat contact area.
I do remember, however, hearing many people saying they used unleaded in pre-1975 cars with no ill effects. If someone knows why, I’d be interested.