Tetraethyl lead was developed by Midgley working for GM, it was one of many compounds tested to prevent the knock.
It was marketed as Ethyl so that lead wasn't mentioned.
As leadsled(Honda GoldWing?) said it morphed into a cure all, preventing valve seat pounding and tuliping of the valve head and lubricating the valves.
As for preignition I would be looking at ignition timing.