My family has been in the fuel distribution business for more than 40 years, currently my brother is the general manager. They have always been distributors of one major brand. As other have said his gasoline tankers will fill up at the rack lined up with tankers from many other companies however they do add their required additives for that brand at the rack during fill up. Now whether those additives enhance performance he cannot guarantee but he will tell you that at their branded stations are checked regularly by the national brand and those additives better be at the correct level or you risk your contract. I imagine other national brands are the same way.