Over the years I've heard different opinions on this but, just in case anyone is interested, I had the facts confirmed last week. I spoke to a woman who has worked in the lab at the Grangemouth refinery for thirty years. The base fuel bought by the supermarkets and the big brands is identical. Where they differ is that the big brands add other products to the fuel later. Also, budget retailers tend not to be as rigorous in cleaning their tankers and fuel storage tanks so you are more likely to get contamination.