Had my car since it was 2 year old ... it's now nearly 11 years old and 95% ish of my fuel is Tesco's .... no problems whatsoever.
My missus nearly always uses Asda fuel ... no problems there as well ....
TWR //each service the plugs were as clean as the day they were put in, my friends noticed his cars plugs were Sooty & the car never run well on "Tick Over//
Hardly a scientific test, the only fair way would be to run the same engine for say a month on one, check the plugs and then run for another month and ditto and then make a comparison.
All fuels come from huge complex refineries, I can't see them making a special "brew" for supermarkets.
"All UK fuel must meet British and European standards - EN228 for unleaded and for diesel EN590.
Fuel companies do often add a cocktail of additives to boost performance, but this is really where the difference starts and ends.
For this minimal privilege, you will pay a hefty premium. And fuel, at its basic level, is generally the same in terms of performance whereever you buy it."