Local racing legend, George Nitta, goes out and tests gas every week at various stations for his automotive radio show. He uses a Hydrometer to test octane and claims Shell consistently has the best gas. FWIW, his tests also show 92 octane is really only 87-89 octane. I don't know how accurate a hydrometer is but supposedly NHRA uses it...Me, I mostly use Tesoro or sometimes Shell because both are close to me. I find no difference between the two...
For the life of me I can't remember what Base the guy said it was on. He followed me into the gas station one day to talk story. He lives in my area maybe I'll run into him again one day.