Well once i had to use supermarket diesel, the same trip on Shell or Esso has always been 49-51mpg. I've done this trip about 50 times, 13 miles from east london to west London 8am Sunday, so hardly any traffic. The supermarket diesel gave me 45mpg. So is it inferior? The fuel ran the car fine, if not a bit grumpier, though diesel engines tended to be grumpy anyway. Supermarket fuel got me from A to B and the car still works...not sure about watered down petrol, but maybe just plain boring petrol without any aditives. So if all fuels are the same, companies like Shell and Esso have been telling us one big lie regarding their R&D and 'superior' fuels. I do believe thogh, the base petrol is the same (like beer is all the same), perhaps supermarkets do not add anything to it, where as big fuel companies do.