not really a dead horse, but 91 octane is 91 octane is 91 octane. Does that answer your question? as far as I know, there hasn't been any testing done to see what difference the small amount of ethanol makes in gasoline. the thing that makes we think the E10 would be better is that ethanol is oxygenated, so even though it's probably a small difference, the car probably will make more power with the E10.