Hi Dave and welcome to the club. I have never had problems with supermarket fuels, but I have been advised that the additives put in by the major suppliers - Shell, Esso, BP etc do make it superior and can give a much better performance. I put in a tankful of standard Shell diesel last week[ a Shell Garage is 500 yards away ] and did my normal local drives -- supermarket, into the town centre, and picking up the grandchildren from school, with the longest run being a 40 mile round trip. My car [ 2.7 diesel ] did 23.9 mpg. I had been getting 28 mpg with Shell V Power. Today I had to go to Watford, a 200 miles round trip on a very busy M1 and M6 with some static traffic at rush hour. I was rather pleased with 46.4 miles per gallon --- I had topped up the tank which was half full, with Shell V Power! I do think the more expensive stuff makes a difference, and at the moment I am getting between £8 and £10 every three month fro Shell Drivers Club which makes a bit of a difference, Regards, Peter.