Because of the SC, I have to use premium. Each station seems to have a different "premium". Some Marathon'*, Shell and BP (Amoco) use 93. Other Marathons (Speedway), Citgo and Sunoco use 92. Of course for an extra dime per gallon, you can get the 94 at Sunoco. I don't know of any stations that use 91.
I have not noticed any difference between 92 and 93 yet, but usually try to get 93 when I can.
Actually I go out at night to get my gas so I never know what the person put in the tank. I usually shop at ricer cars they donít need the gas they need a clue. ( for those with a misunderstanding of my warped humor please remember I am joking)