One major difference brand to brand is quality control. I once had a presentation on Standard Stations (Before they became Amoco and then BP) and in fact did a college paper on Standard Oil myself when I was at Cornflake U.
Standard had some rules, one of them concerned the amount of non-gas in the tanks (normally this means water) Their inspectors would check the tanks and if there was more than a specific amount of water in it, they called in the pumpers and drained the tank.
The state of Michigan has a law which also sets a maximum amount of water in the tank, Almost exactly 2x what Standard allowed
Now, some brands (Long gone from the Michigan scene) Well, I won't name the brand or the station because as I said, it's not here any more, so I won't be providing warning.
One customer took his car in running on fumes... Filled it up,, it ran poorly He found water in the gas, FIVE GALLONS of water in the gas Now the tank holds about 18 gallons and has perhaps 1-2 gallons in it when it's "empty" so that means at least 3 of those five gallons came from the station.
Neighbors said the manager often took the garden hose to the fill ports for his underground tanks.
That, is a quality control issue... And may well have to do with why I don't see those stations much any more
Another true story, My daughter's Godfather (Who of course is a good friend of mine) used to fill up at an "off brand" station near his house. I filled at the Amoco near me (short side, the off brand was a chain, one day as I was filling up at my local Amoco I watched a fairly attractive young lady at the other side of the island filling her Corvette, she did a very professional job, I commented on it, her reply was she managed the off brand station down the road. Same off brand as my Friend used)
He used to tell me how I was paying too much for Gas and how much he was saving.
He switched to Amoco after r the water in his gas tank froze and broke his $300 fuel pump
So, even with laws equalizing the gas,,, Quality control is up to the oil company, There are some brands I tend to avoid