Proghound
Admin Team
As far as I know the additives for the brand are added neat to each tanker, in the correct quantities for its laden volume. Not sure at exactly which point this happens, but it would make sense if it was at the tanker depot when they are empty, before filling up with base fuel stock at the refinery. Not least because they would then definitely be mixed in properly before delivery to the brand's fuel stations.There are six major and one minor refineries in the UK; of these only one is owned by a known brand - ExxonMobil at Fawley. But do you imagine that an Exxon petrol station in Scotland is supplied by Fawley? No, any petrol station will be be most likely supplied by the nearest refinery. Any special additive would have to be added at the refinery but that would mean that every customer of the refinery with a special blend would have to have their own storage tanks. How likely is that? For "wizzy additives" read "marketing" I suspect!
RAB
There have been incidents of the incorrect additives being added for the fuel in the tanker (or possibly vice versa). A few years back there was a cluster of deliveries of petrol containing diesel additives, which caused a lot of lambda sensors to be eaten away in the areas local to those fuel stations, as well as a few further afield. That was Asda & Morrisons' fuel if I remember correctly: even the supermarkets (or at least the oil companies with the supermarket supply contracts) are putting some level of additives in their fuel. I suspect these are just the bare minimum required level of detergents to keep fuel systems from harmful levels of clogging up, whereas the brands are using greater quantities/more effective cleaners, plus anything else they throw in for burn optimization and component longevity. The latter two of these qualities still have plenty of scope to be marketing hype though, due to the difficulty for individual customers in observing their effects.