I'd like to say that it is ethanol that is added to gas. And you actually can get more power from ethanol because it has a higher octane rating than gasoline(ex. modified GTRs are tuned to run on e85 to produce more power than 93 Oct; as well as the koenigsegg agera R). Ethanol has less energy than gasoline so fuel economy suffers.I think 91 would be fine. I think the bigger issue is added alcohol in the gas. here in the Midwest it hard to get straight gas without some methanol in it.
You may get a little less power due to the ECU adjusting to the lower octane, but the car should adjust itself.