The manual recommends 93, but don't go down to 87 or you'll end up in "knocks"ville. The only gas I can find around here is 92 and mine runs fine with that. I just throw in a can of octane booster every 2 or 3 tanks. As for mileage, my M6 gets around 18-19 with fairly aggressive driving(ok, sometimes too aggresive), mixture of highway and city driving. But I did just go to Seattle and back today, 200+ highway miles and a few in the city, and averaged 24mpg. Aside from poor dealer prep, paint is the most common issue I've heard of or experienced. If you want the full scoop go to
LS1GTO.com and post your questions. Lots of 04-05 owners with plenty of experience. You'll probably get a lot of responses saying "Just go and buy the damn GTO", but you'll also get a lot of level-headed replys with good information and insight. In the end, especially after driving both cars, you'll most likely just go and buy the damn GTO.