One detail is that the EPA's excessively stringent smog metrics require the addition of oxygenates (https://en.wikipedia.org/wiki/Oxygenate https://en.wikipedia

One detail is that the EPA's excessively stringent smog metrics require the addition of oxygenates (https://en.wikipedia.org/wiki/Oxygenate https://en.wikipedia.org/wiki/Oxygenate) during the summer (when the US gasoline market Balkanates into close to 20? separate regions, each with its own blend). Refiners used to use methyl tert-butyl ether (MTBE) but that was declared icky in comparison to ethanol.