Rhetorical question of course. Save specialty fuels, pretty much all retail base fuel is a fungible commodity that's made, traded, transported, and stored as a fungible commodity. Of course there are differences in base fuel depending on any number of factors, but it's not as if the fuel made by Chevron, Phillips, Sunoco, Shell, EM, or Valero is appreciably better or worse.
Still, I think it's been established that the primary difference is the additive package in the fuel delivered to the end user. Costco has some of the lowest prices with a strong detergent package. ARCO is cost competitive, but probably meets the minimum Top Tier requirements. Still, even with the big brands there seems to be a huge difference. I was looking for gas today and came across three stations close to each other. Shell was $3.159/gallon cash for regular. Valero was $2.579 as was Exxon. I thought that Exxon was supposed to have an additive package comparable to Shell. However, Exxon and Mobil are just returning to the San Francisco Bay Area and their prices seem to be very competitive right now.
Still, I remember a friend who was convinced the Shell and Chevron were the best, and Exxon or ARCO were [censored]. I don't know if he went on anything other than marketing.