Fuel is all the same, only when it gets put into the tankers at the dustrubition terminals is "addative" added into the "blend". Without these addative's, believe it or not, petrol WILL NOT work in a modern car.
The difference in petrol prices between the "big boys" and the supermarkets is due to The amount of blending (addative) that is added at time of loading.
All petroleum producers put additives in their fuel. These are a carefully controlled blend to enhance both the storage and use of the fuel and maintain the octane levels. Detergents added will help to keep the injectors and engine clean. This in turn means the engine is more efficient and you will get better economy.
Other additives may include bio elements such as esthers or alcohol, bio-ethanol.