I have established time and time again, with both petrol and diesel engined cars I have owned over the years, that I get better fuel economy from non supermarket fuel. I am not sure what makes the difference, it must be any additives.
I had a 1.6 1993 Mondeo a few years ago, if I filled to the brim on supermarket fuel I would achieve around 300-350 miles to as near to empty as I dared. If i filled with non supermarket fuel I would get 400-450 miles. I checked this many times, especially when people questioned when I told them, so I know this to be accurate. My 1.9dci Laguna without doubt goes further on non supermarket fuel.
ps All milk does not come from cows!!