It is my understanding that lead was added to gas as an octane booster. It was removed because it harmed the environment which put out the search for another octane booster. MTBE didn't work because it can be smelled and tasted at extremely small amounts. In ground tanks were replaced but eventually the stuff was replaced with alcohol. Meaning that alcohol is an octane booster. My research shows that to be true.
Ski and others: Maybe a moot point by now but just so we're on the same page with this I've copied what I was referring to here, for the sake of context:
"you purchase E10 gasohol with an octane rating of 87, phase separation occurs and as the ethanol sinks to the bottom of the tank the octane rating in the gasoline actually gets lowered to around 82 to 84. This is too low for a gasoline powered engine to run efficiently and causes loss of power and engine damaging pre-ignition."