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My sister ran a station in my town and the truck that delivered to them (Statoil) also delivered to the others (Esso, Texaco and some supermarket ones). Other named trucks even collected from the same place at the docks in the port, where they have storage tanks.
So why do they all have different prices? Any why does the price of a barrel on the stock exchange instantly effect the price of petrol? Is the knock effect that fast?
I don't normally use Optimax but I will give a try to see what it does for my A3.
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Supermarkets often sell petrol at a loss as they can afford to. It attracts the punters in who tend to shop and get petrol at the same time.
As for optimax against normal unleaded, without a doubt, you should feel a difference. Basic engine science says that a higher octane fuel will resist knock better, thus less ignition advance and more power.