the tetraethyl lead was put in petrol purely for the valves of 4 stroke engines it acts by preventing the heat of combustion burning/welding the metal of the head/valve to each other the sealing effect starts to break down and the valve ression starts.
a side effect of it added was to reduce the temp of self igntion of the fuel (higher octane rating) so engine designs to this into account

driving hard will increase this with lead free fuels but nothing will stop it
there are additives on the market but the best option is to find an engine thats designed to run on unleaded fuels as conversions (hardend seats) dont effect compression ratios etc and have to run retarded ign timings and are just not as efficent
the cost of addives that actualy work are expensive so a search for a cheap unleaded motor will pay of in the long run.

As it is only the early Pinto engines that were leaded I would not stress much, your more likely to get an unleaded head, if not a good upgrade is the later unleaded injection head, easy straight swap + 5 to 10 bhp extra just dueto the better inlet port short side turn.