In the old days:
Low cost gas was to be avoided because the cheaper stations didn't maintain their tanks. Dirt and water would screw up your car.
Now:
EPA regulations have forced stations to have seal storage tanks and all have filters to keep crud from getting into your car. All gas comes from the same major refineries. Yes, some put more additive in them than others but, the percentage of Techron in Chevron is so infinitesimal that its more marketing than science. Buy a bottle before each oil change and run a tank of gas after that one before changing oil. Everyone has their own blend of additives but they all have some kind in there.
Oh BTW, as far as Shell's nitrogen enriched gas... 78% of the air your car breathes is nitrogen.
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Joe
See Porsche run. Run, Porsche, Run:
`87 911 Carrera