You'll destroy the pistons with 91 octane in time. It's funny how many people do it and then sell the car with the "never been raced" sales pitch.
If you get 116 octane fuel to set in the garage just dump in 3 gallons to be mixed with 10 of the 91 octane pump and it will be around 97 octane. Done.
I most often used 100 octane unleaded from the pump here in Scottsdale, AZ.