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Methyl tertiary butyl ether and California used to be tight like Walter and The Dude. But then MTBE got all overzealous and screwed up the mission by polluting the groundwater. So Cali partnered up with ethanol, but you know, things with ethanol weren't really working, either, as it actually increased air pollution in some ways. So finally, after a bunch of pestering, the Feds tossed off an "Okay kids, have it your way," and let go of a Clean Air Act mandate to force states into using oxygenates in congested metro area. Plus, y'know, we may even save a buck or two per tank.

EPA lifts requirements for states to add ethanol, MTBE to gas [Sacramento Bee]

Mmmm Thirst-Quenching Motor Fuel! Shell, ExxonMobil Create Cleaner-Burning, Non-Lethal Fuel