Going back to around 2003 I recall that I had poor running problems with my car (at the time it was a Landrover Freelander Auto) and complained to the dealer. His response was to ask what fuel i was using, to which i responded Sainsburys.
He immediately suggested that I change fuel to one of the main fuel companies (is this case BP) and see what the difference was.
I was very dubious about the possible result, but I have to say that there was a significant difference ie, much smoother running and better MPG and I have never used supermarket fuel since. OK it was a penny or two dearer, but this was more than offset by the increased MPG, probably in order of 10%, which of course on that vehicle was only 2 or 3 miles different.
So, i'm now on my fourth Jazz, and I still stick with the main petrol companies wherever possible.
I suppose I could be classed as a somewhat spirited driver, however my Mk3 EX CVT has covered almost 7000 miles so far and returned 52.7 MPG on the trip computer, and as long as it stays on the plus side of 50 MPG, I'm happy.