I guess most people are not aware that all gas (in the area you live) is created equal. This is because for each area of the country their is only one refinery making the gas. This is especially true during the Summer, due to the EPA requiring at least 18 different formulations for at least 18 different parts of the country. Twice a year all refineries shut down for about 2 weeks so they can reformulate per EPA rules.
Here in Florida almost all of our gas comes from one refinery in the Gulf coast. Now the only difference between brands is that when the gas is moved from the refinery to storage tanks Shell, Mobil or whatever brand can then add additional additives if they want to. Otherwise all the gas (regular, Plus and Premium) has the same cleaning additives that the EPA requires. Also the Refinery only makes Regular and Premium. Plus is made by just blending Regular with Premium either at the retailers storage tanks or at the gas station.