I used to say that they all buy from the same refineries. But now I question that view. I always avoid scruffy stations that look like their tanks might be rusty or contain water/contaminates.

Now worry that cheap gas might be because additives are being deleted to lower cost. Check this out.

http://www.toptiergas.com/retailers.html

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2007 X5 3.0 si Black 'n Tan
2004 Boxster S Black 'n Tan
1970 914-6 Black 'n Tan