I've been using Sunoco ultra 94 octane for years...they recently stopped selling 94 so i now use their 93 octane. When my buddies and i used to track our cars and even the X, Sunoco used to have even higher octane levels at select stations. Never had any problems.

However, i did nearly run out of gas once on a snowboarding trip and my only choice was a nearby WAWA...not even a mile after filling up the SES light came on. The guys at my mod shop suggested that i drive the X for a while to burn some fuel and the sensors might adjust themselves. Sure enough, the SES light went off and i've never used WAWA gas since.