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91 octane is actually technically oxygenated as well as the 10% ethanol content allows the companies to sell it as such. there is a marginal performance difference between 91 and 93 due to the alcohol content (alcohol is a cheap easy way to boost 87 up to 91 without adding the nice petroleum distillates, and fuel additives you get with 93, while at the same time reducing specific energy output by 1-2% due to that same alcohol) 91 is the gasoline equivalent of E85 and shares it's same fault only smaller, and doesn't carry the 100+ octane rating of an alcohol base blend. The 91 octane sold in the state of California is even worse dues to it's requirement to further reduce emissions. Gasoline mixes also vary by region, and time of year as well as by brand. You may like shell in spring, B.P. in summer, Mobil in fall, because each company varies the amount of additives in they specific blend they order, and many companies actually sell the exact same gas purchased from the local oil distributor, some times you can even watch a tanker pull in to one station unload fuel, cross the street and put the same gas in the competitors tanks. This time of year i have to be careful with roadtrips to no-snow states because on the drive home if i have that fuel left in the tank there isn't enough antifreeze in it once you get back into sub 10degree weather.

Edit:Many places now also sell a 10% ethanol blend in 93 octane rating as well so that is something to look for because in many states companies are not required to actually state what is in the fuel so long is it is in quantities less than 10% and meets the advertised octane rating