Just to add a bit of sanity to this thread, all petrol is made to the same standard, if fact in most areas it comes from the same refinery, no matter the brand, orthough the supermarkets and cheap brands may also buy unbranded petrol on the spot market, oil version of wholesale.
The difference is that the top companies put more additives in it, to clean the engine and some of these may cause a slight improvement in performance and economy.
The last ,bad, supermarket petrol problem was I think that a batch had the diesel additives put in it, the engines did not like the large amount of anti-foaming agent in it.