Besides raising the octane of gas,tetraethyl lead was found to improve the heat transfer from the exhaust valve to the head.Exhaust valves can run red hot,when they do,micro welding can occur each time the valve is on the seat for that very brief time.After a time,the micro welding errodes away the valve seat and or valve.Lead,or valve seats made of errosion resistent materials lessen this.
On many old engines the carburation is on the rich side which keeps the valve cooler with less micro welding.52 PU suggested loosing the valve clearances a bit so the valve stays on the seat longer,tranferring more heat to the head and guide.
Mystery oils added to the engine oil won't do a thing for valve seats.Most farm gas tractor engines had valve seat inserts or induction hardened seats as does many heavy duty engines designed for continious full throttle service.
