With two major supermarkets with filling stations within half a mile of our house I always use supermarket fuels, diesel in my case.
I drive two vehicles, a 2009 Kia Sorento 2.5 XS crdi and a 2013 Ford Fiesta 1.6 eco diesel Titanium X.
Supermarket fuel notwithstanding, neither of these vehicles, which have fairly high mileage, ever misses a beat, both start first time every time and both cars pass the MOT year after year with no advisories.
I'm not obsessive about measuring mpg, but the Fiesta is very economical and the Sorento gives no cause for complaint when towing our t/a caravan or driving solo.
The reliability and continued longevity of both vehicles is i.m.o. due to regular servicing and maintenance coupled with driving techniques that lead to good fuel economy.
There's an Esso service station within a mile of us, and on the very rare occasions when I've filled up with branded fuel I can't say that I've noticed any difference in the way that either car runs.
I've been using supermarket fuels for at least 25 years with no adverse effects on any vehicle that I've owned in that time.