And remember, the lead was tetraethyl lead, a costly additive mostly to raise octane.
The lead added gas was called "Ethyl" for obvious reasons or hi test and in the rural areas NOBODY invested in hi test for their working class stovebolt.
So you might say these engines were designed for unleaded fuel.
Then again I never knew anyone in our circle that saw 150,000 miles without opening the engine for repair.And very few cars were run to 150,000 miles
Poor oils and not enough servicing would have most down for repair before 100,000.
In my opinion, excessive valve train clearance, noticible by excessive noise is more harmful to valve seating surfaces than lack of lead.
