I use 93 octane Shell or BP generally and I am very pleased with the cars' (both of them) performance and I have not experienced any carbon build up on the exhaust tips. They refine Marathon, formerly Ashland Oil, in Eastern KY. and I can tell the difference when we use it as there is a slight hesitation before hard acceleration. It is only available in 87, 89, or 91 octane. The difference is more pronounced after a couple of tanks of the lower octane. I am willing to pay the difference.