Wrong I am afraid.
The refinery doesn't mix additives into fuels, a refinery only refines crude oil into its constituent parts.
ALL, additives are added at the tanker not before, not after. In actual fact fuel, or sprit, as its know on fuel terminals won't actually run a car anyway. Once the sprit leaves the holding tanks, it is pumped into the lorry, where all the additives are added.
How do I know this, Well, I service the additive plants on fuel terminals across the UK, and trust me, the stuff they add is not pleasant. It burns through steel toe capped boots within an hour, rots material and burns your skin within seconds. VERY NASTY STUFF.
Also rest assured, there is NO difference between any fuels apart from Tesco 99 ron. The amount of additive is exactly metered into lorries cargo, whether or not its going to sainsburys, BP,Esso Asda etc.
There is a lower limit to how much additive is added and a higher limit, but it must be dosed between these 2 limits from the additive plant, outside of these limits, there is an automatic cut off for the lorry loading.
Trust me, all fuel is brewed in exactly the same way.
not true, I have heard that both supermarket plus fuels are mixed differently to BP or shell. The additives are literally added in the tanker rather than at the refinery so can be a bit hit and miss and not as well made with less additives. Sainsburys is sourced from BP but that doesnt make it the same as BP ultimate.