I think there's a sticker inside the fuel refill door that says "High Octane Recommended" or "Premium Fuel Recommended".
I have 9,500 miles on my 03 GT and have used nothing but Exxon 93 Octane. I have been happy with the way the car performs - I have an automatic (ugh, I know, first time ever) and I can chirp wheels any time.
I can't compare to "lesser octanes" since I have only used 93, but as long as I get that feeling of being pushed back into the seat when I wot, and crushing rice jalopys with the 3" pipes out of the street tree, I'm happy. And I do.
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