The octane specification for the 500 is 87 minimum, 91 recommended. Around here, the typical choices are 87, 89, and 93. I've been using 87. Occasionally, when hot, the car will idle a little rough, but that's the only issue I have and I suppose it might be helped with higher octane fuel. It would cost me another 50 cents per gallon to go to 93. What do other people run?