Which is apart of why I suggest finding what works best. Gasoline differs across the country, region to region. Quality is better in some parts than others. Some is conventional without ethanol as an oxygenate, some with, some reformulated, some reformulated with ethanol as an oxygenate...
You have to remember that there's alot of trading and alot of exchange business going on with refined basestocks. As much as they are rivals in the market, they are also dependent on one another. Each of them don't have the assets or infrastructure to physically market their refined products in all 50 states. But, they trade and sell and exchange barrels to maintain a market presence in as many areas as possible. It's lucrative for both parties. It is a model that works. And it means you may be filling with Shell branded gasoline which originated from the same basestock, same refinery, pipeline, terminal and product tank as the Exxon station across the street.
And this isn't even considering the variables introduced after it is released from a terminal...