Avgas is 100 Octane. The octane wouldnt necessarily make it run shitty, but the lead will. Avgas is leaded to help lubricate the valves, at least thats what i've been lead to believe. I used to only run premium in my old volvo turbo. Switched it to regular after Katrina. No change in mileage or power. No detonation. Ive been averaging bout 27mpg in my 5spd fusion on regular. Unless my mileage goes up to 30+ around town I see no need to run premium