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I like to alternate gas, because the detergents clean up after one another.
QFMFT. I was told this once by a petrochemical engineer for Amoco (now BP). If an employee of an oil company is giving me a reason NOT to buy his company's product all the time, I'm gonna listen.
As for brands, 95% of the time it's Shell, Speedway or Citgo, because that's what's accessible near my house.