My brother was an Exxon dealer in another town for 25 years. I had been buying my gas at a local chain of unbranded gas for years. However, I had to use carb cleaner every so often because of the red varnish that would build up in the carb. My brother said to clean the carbs, switch to Exxon for months and check the carbs again. I did and they were squeaky clean. No more unbranded. One night I was coming home late and stopped to buy gas at a Conoco station. Up until then I had switched to Hess because it was one block from my house. I had been noticing a little hesitation and also knocking on hills. By the time I got home my truck was purring with no more knocks or hesitation. That was 15 years ago and since then I rotate tanks of Shell, Exxon and BP, all which are close by. Last Friday I paid $2.99 at a brand new Shell station just off the interstate. There is a difference. Maybe some people aren't as sensitive or aware of the way their engine runs on different brands.