I agree with what
OneQuartLow said. Gas quality appears to vary locally/regionally. I've heard numerous people claim that tanker trucks of numerous liveries have topped up at the same pump at regional distribution centers.
I think that if your car is picky, you should try to find a station near you which sells gas your car/truck likes and stick with it. Same brand gas across town may not perform as well. Some local stations may also have old, contaminated tanks. These should be fairly rare in most parts, though, as older steel tanks are replaced with double-wall fiberglass tanks for environmental reasons.
Around here, Mobil has had repeated problems. They've had really crappy gas which was contaminated so bad that it was clogging fuel filters and actually made the news.
Also, I had a 1990 Integra that would run on anything ... except Mobil and the cheapest gas from the least reputable-lloking gas station in town. The type of place most people on this board wouldn't even
think of pulling into.
When I filled up with gas from these places, the car would knock and ping badly. 87 octane from any other station (Hess, BP, Sunoco, Gulf, Getty or Citgo) was fine. The funny thing was, Mobil has always been the most expensive in this area and I wouldn't use it if it was the cheapest.
Now I use Sunoco (mostly) with occasional fillings at Gulf stations as well as Hess. I don't notice a difference with my car. I use additives so frequently (formerly Red Line SI-1, now mostly Neutra #131) along with isopropyl alcohol, I'm not really worried about any injector fouling.
We don't have Chevron, Unocal 76, Phillips or Shell in this area.
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Bror Jace