I have a stock 2011 Gt premium I bought in March. I went to the gas station today to fill it up for the second time, only had 365 miles on it. I intended to put 91 octane in as that is what is recommended. Apparently my choices were 87, 89, 93. I put in 93 octane. I'm not asking because i think damage will occur, I'm just curious. What do you use? Obviously if you have an octane specific tune I'll assume you use that, but what if you can't get the specified octane, is it better to go with increased or decreased octane fuel?