I've received the same e-mail several times lately, calling for folks to boycott the major oil companies in an effort to force some price lowering. After some consideration, I decided to share some information that might be helpful to you. I worked 6+ years for an oil distributor. Here's the sad truth.
There are "branded" stations and "unbranded" stations. A branded station clearly displays an oil company's name.....this is largely for credit card purposes. An unbranded station carries no oil company name and accepts only major credit cards.
Even if you could keep up with buy outs and purchases to know who truly owned a company, you'd be hard put making sure you weren't buying gas from them anyway.
Some branded stations accept "drops" from only thier company's rack. (A drop is a fuel delivery. A rack is what the tanker trucks load the fuel from.) Others allow drops from different racks.
As for unbranded stations, well their gasoline could come from any rack at all and could vary load by load as the distributor probably bids whichever rack is cheaper that day.
Just wanted to mention it for what it is worth.........do whatever you feel you've got to do.
As a side note, did you know that there is no "plus" or "mid-grade" refined? They achieve this product by mixing x number gallons reg. unleaded with x number gallons super.