I've kept a log of every fill & my fuel usage since I bought my car. Averaging the figures over a 10 fill cycle for each brand there is no difference between Shell, BP & Caltex. Also I can't notice any difference in the way the vehicle performs. I usually try to stick to Shell (brand loyalty), and try to get my fuel from the same servo's when I can (ones that I have had no issues with, and I'm confident that they don't dilute their product with regular unleaded or Tolulene.)

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The older you get the more you realise that no one has a clue. Every one is just winging it.
MY05 STI