Where do you think the supermarkets get the fuel from? Shell, BP etc.
Obviously fuel comes from the same place, HOWEVER it is the additives which is put into the fuel after it is made that makes it cheap supermarket rubbish or top brand Shell/BP.
Supermakret fuel does not have the same additives in that shell etc does, it lacks lubrication factors, doesnt burn as well etc etc.
To think fuel is fuel is very short sited, do a bit of searching around on scentific sites (i.e not opinionated sites) and you will see what i mean.
Just think of it like this: Years ago there were fuel ratings, fuel stations supplied: 1 star fuel, 2 star, 3 star etc, some cars would not run on the basic 1 star fuel, as it did not have the right additives in, however some more basic engines would. So you had to pay more to get the better grade.
Treat Tesco/supermarket rubbish as the 2 star fuel from years ago, and the Shell/BP as the top 4 star. See where im coming from now?
There has been numerous reports about supermarket fuel and top brand, most say their car doesnt run as well on the cheaper stuff and they get less MPG.
I would never use supermarket fuel, is it really worth it to save a few pence??? Shell is exatcly the same cost as Tesco etc round here, so i dont lose out anyway.