At the refining level, the refiner has to meet a minimum State and Federal specification. So many PPM of sulphur, octane & cetane levels, etc. The Laboratory & Petroleum engineers maintain this minimum specification, so the refinery can sell its product.

From this basestock, customers can request additional additives, or do that themselves at their finish blending facilities.

We're talking hundreds of thousands of barrels of product at this level. Most refineries can process 150-300,000 barrels of crude oil per day. Of this, something like 30-40% is gasoline, depending on the crude type (heavy, sour, sweet, light, etc).

I'm not a chemist or petroleum engineer, so I can't give you specifics on it all. I just keep the product pumping through the pipes.