The only problem is that different areas have different gas chains. For instance, no Arco, 76, or Olympian here. All Texacos here are now Shells (which will eventually happen everywhere) and we just now have BP after they bought Amoco. Then we have Exxon, Chevron (Rare), Mobil (same as Exxon), and increasingly a chain called "Liberty".
I know steviej will chime in on this so I'll let him explain it but basically all brand name gas is pretty much the same, with a can of additives added. Generally its best for the motor if you skip around gas types.