It is widespread. I was on another motoring forum where there was a guy who was in to getting good economy from his diesel. He claimed that he was getting around 10% better mpg by avoiding supermarket fuel. Many were skeptical. Those of us that actually put his theory to the test, including me, got the same results - about 10% improvement. I was doing long distance commuting at the time, so was able to test as accurately as possible, by virtue of doing identical journeys week in, week out. Three consecutive tankfuls of Sainsburys v 3 consecutive tankfuls of Shell, and I consistently got around 10% improvement tested back to back with all other variables reduced to insignificance. As the Shell was much less than 10% more, the Shell gave me lower costs overall.
The reason why so many people are unaware of this is that very few actually go to the bother of accurately measuring just how much fuel they have used, particularly when they are skeptical in the first place, (as was I) and a 10% difference is not particularly noticeable unless you actually go looking for it. Therefore most people carry on in blissful ignorance thinking they are saving money using the cheaper fuel.