[The Art Doctor]

Technically the ethanol is anti-knock at 110 octane by itself that is why its run in race cars with high compression. For non-ethanol fuels common "rocket fuel" additives back in the day were benzene, toluene and xylene which all are around 100-120 octane individually. When they were cheap guys would add a gallon of them to the tank of regular to make high-test in areas where it was not available. Now toluene is regulated and hard to find and xylene is over $25 a gallon so its just cheaper to buy premium from the pump rather than mix your own. The octane boosters in a small bottle are nothing more than benzene (naphtha) and a touch of xylene at a huge mark-up.