Ethanol has tax incentives that can make it advantageous as an octane
booster even when oxygenation is not the primary goal.

a commonly used farm fungicide -- available in 1981 for $70 for a 55-gallon
drum, plus shipping, quantity one -- works far better on an ounce by ounce
basis. I seem recall GM's research indicated that 1 ounce of that fungicide
increased the octane rating of a gallon of gasoline by about 5 points.