TEL (the "lead") is added to a gasoline to increase its octane rating or resistance to detonation. It is not added for any other purpose. The TEL doesn't alter the combustion characteristics of the fuel, it's there to prevent detonation from occurring. The actual amount of TEL in the fuel doesn't matter providing the fuel meets the octane quality specified. (lean rating >100 MON, rich rating >130 P.N. in the case of Avgas 100LL). So if higher octane blendstock is used, less TEL is required for the fuel to meet the octane requirements.