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I don't know how it works across the pond, but in North America, the supermarkets will buy the gas from whichever company can provide it for the cheapest at the time. One month it could be supplied by exxon, the next month, shell.
The only possible difference in quality is that in order to have the lowest price when selling to the supermarket, some suppliers might not include the same additives that they do for the branded gas.
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