I would not trust the big Oil companies (EXXON, BP, Shell) so much. They are hugely profit driven, and Shell especially really hypes up themselves in their commercials. At the end of the day, we really don't know what beneficial additives they add into their gasoline.
But I do agree with you, that cars can run different on different brands of gas.
As a consumer, and even with the internet, do we really know what corporate America does with their products? Sometimes we do, but often times we're left wondering about the hype that we just viewed on the last commercial.