Actually, the old style “lead”- tetrethyl lead- was partly an octane booster but primarily a valve face lubricant.
Under high-load conditions, the face of the exhaust valve can get nearly-glowing hot. As the valve slams shut, it actually sticks to the seat- microscopic areas basically get “welded” together between the two. The next time the valve cycles- a fraction of a second later- it tears out those microscopic welds.
What this causes is seat erosion. The face of the combustion chamber, that the valve seals against during firing, wears away, and the valve itself receeds further and further into the cylinder head.
After 1971 or 1972 or so, most auto makers were using one form or another of “hardened” valve seat, which better resisted the erosion.
If you have a pre’72 classic that you drive regularly, a small bottle of any of the commercial “lead” replacer additives- true tetrethyl is illegal as a motor fuel additive these days- will help prevent the seat erosion.
GuitarDave: You have that backwards. If you run unleaded in a car designed for regular- meaning made before '72 or so- you’ll suffer no problems whatsoever. Unless, as I mentioned above, you run it for a long time, like 50,000 miles, on nolead in which case you’ll see valve problems occuring.
However, if you add leaded to a more modern car with a catalytic converter, exhaust gas temperature sensor, or exhaust oxygen sensor, those items will be essentially rendered useless within one tankful of leaded.
In all cases, the lead coats the sensor or the catalyst mesh in the converter, after which they no longer work properly. And no, there’s no easy way to clean the lead off- the parts must be replaced.