That list looks pretty flaky. Exxon is definitely a "top tier" supplier and BP owns Amoco, long considered to be one of the best refiners in the industry.
Most of the gas resellers like Wal Mart and Costco buy gas from the major oil companies, but since they shop for price, you might get Shell one day, Exxon the next. The differences between the major suppliers is generally in the detergents they add to the gas. Detergents keep your fuel injectors clean, but just about all the major suppliers add excellent detergents to all brands of their gas.