This is true and not quite true.
It is true in that the base gasoline is the same. For a given geographic area, there are huge tank farms that contain the same base gasoline in mulitple grades. All the tanker trailer trucks pull up and fill up there. They then haul this gas off to a little filling station near you.
It is not true in that the different brands of gasoline (Mobil, Shell, Sunoco, etc) usually have a concentrate that the truck driver has to add to the tank when he fills up the trailer. This concentrate then mixes with the gas when the tanker is filled up and from the motion of the vehicle. The concentrate, however, is specific to each brand. Each company requires/provides their own "mix". That mix is proprietory information. Usually detergents and other stuff.
How do I know this: my dad does this for a living.
steviej