All fuel in this country is to a very high standard.
There is absolutely nothing wrong with "super market" fuel. The only difference is that it is bought wholesale to make it slightly cheaper. It will be to the bare minimum to the standards. But standards that are high to start with etc.
It is true that some brands of fuel do add more additives, but the amounts are tiny and only over a very long time would these differences be noticed. There is so much more to go wrong with a modern engine that any slight benefits doesn't make a difference with the higher priced paid for this "premium" fuel.
Higher octane fuel does help a engine run smoother with slightly higher power if the engine is programmed to use it. It can make a few BHPs more, but most engines will hardly produce any more power. Making sure your tyres aren't under inflated would gain more tangible power/fuel economy.
There are hidden benefits to using super market fuel over any other independent garage fuel. One is how often the fuel is replenished. Busier garages change the fuel more often. Quieter garages have to store the fuel longer, and premium higher octane (expensive) fuels stand even longer.
This creates a problem of moisture mixing with the fuel. There are things in place to help with this, but they can fail.
Modern cars can deal with a certain amount of water in the fuel. But ultimately/long term, it will effect the engine.
If people can actually tell the difference between regular super market fuel and branded fuel stations regular fuel, then this would more indicate a problem with your engine. Your money would be better spent on a full service than the dearer fuel...
