I actually work in the petroleum industry for a company that tests fuels nationwide for all of "Big Oil". A common misconception that people have is that one company has better fuel than the next. That is both true and false at the same time. All of these companies buy and sell fuel to/from each other. The biggest difference is the additives they use in the fuel. For gasoline, the best premium gasoline you can get is Shell. The detergents they use clean the valves etc better than any other. I actually went to a presentation given by a 3rd party (not Shell) about this. Chevron came in a close 2nd. Mom and pop fuel stations do not use the additives that Big Oil does, hence the slightly cheaper price. Another thing is that mom and pop shops do not have the oversight programs Big Oil does, meaning they can get away with a lot more funny business if they wanted. Although private owners own Exxons, BP, Shell etc gas stations, Big Oil has oversight programs to randomly test the octanes to make sure there is no monkey business.
Diesel fuel would be also best bought from a known oil company fuel station for the same reason, additives and detergents. Now that I just bought a diesel I will do a little research and advise who has the best and report back.
Little long but hope that helps shed a little light on the situation