I'm not positive about this but I talked to a guy in an exhaust shop and he was telling me how the cheaper gas companies have different standards than the upper priced companies and only need to have a certain percentage of gas in their gas like 60% and the rest can be any number of other things such as water. he said shell chevron 76 and some others have a higher standard. and that you wouldnt believe how quick the cats go bad(after 2 years or so) on cars that are using the costco gas and 7-11 gas like arco. i dunno tho because i usually use chevron