Additives are added at the supply terminal where the fuel distributor "picks up" product from the supplier. The additive is injected into the delivery stream of the fuel product while the tanker is being loaded.
To make a long story short, motor fuel is refined by the refiners or imported into the US as a product that conforms to an ASTM specification. The product is inserted into the pipeline system in batches. ExxonMobil or Shell or BP or any of the major refiners will put a large volume of their product into the system. The product travels through the pipeline and comes back above ground at wholesale terminals throughout the US. The product Exxon puts in may not be the product Exxon pulls out. If a refiner puts in half a million gallons they have the rights to pull half a million gallons out. Because all fuel is refined to the same standard, it really doesn't matter. There are quality control checks throughout the process to ensure product integrity.
The proprietary additives that are blended into the common product are the difference. The value of the additives are up to you.
My two cents - purchase fuel from a site that prices competitively and frequently turns over their inventory so you don't get old gas. I like modern (recently built) sites that go through a fair volume.
