I can tell you what I know..... Which is that it used to be the case (might still be) that sometimes supermarket chains would purchase petrol on the spot market....which is to say they would buy a small tanker (ship) load or a storage tank of refined fuel on the open market. Usually in the UK, the purchase would be from Holland. The ship would come to the UK and unload the fuel into rented tank space at a refinery where it could have additives put in or not, have the statutory tests done, pay the duty and tax and then be delivered to their stations in rented bowsers (road tankers) At other times, they will buy UK refined petrol. Additives are put in according to the retail customer's requirements at point of sale. So....its not so much that supermarket fuel is better or worse than big brand fuel, its that you don't know what you are getting apart from that it meets the UK minimum standard.