For a long time, Shell was considered the ultimate in gasoline. But in recent years, the consensus seems to be BP. There are places that do tests on this stuff (like a Consumer Reports, etc.). Honestly, I am not even sure if Shell 91 is all that common where I live (Midwest), but I know I prefer to put BP 93 into my 911 and my M3 and if there isn't a BP nearby then I go to Shell in lieu of that. As they say, expensive gas is cheap insurance.