I'm sorry my reply is unsatisfactory for neil.macdonald8. It is verbatim from the technologist working on the unit that produces and blends Shell fuel and optimax. I trust his explanation because I know him and the operators on the plant.
Yes I do work for Shell, and yes we do make a vast amount of money, sadly not as much as we collect for the governement, but we do make money, that is why we are in business and it is why we are good at what we do.
From my own humble experience having driven my IS200 for 5 and a half years I have recorded 52K miles on Shell at 31.6mpg, 10K on Safeways finest at 31mpg, 9K on Sainsbury at 32.8mpg. The best has been BP 33.82 mpg but only for 653 miles. Over 78K miles the average is 32mpg, but I try to be smooth and fairly gentle with the car so Optimax is not for me. ( I'd post the spreadsheet but not sure how)
Bottom line is there is a choice out there, though most of the fuel is produced by a few major suppliers, this is well known and is accepted practice. From my office window I see Asda, Texaco, BP Safeways tankers filling up at the terminal. BUT, you only get the Shell additives on a Shell forecourt. I buy most of my fuel from a company that has it's primary business of petrochemicals. I buy bread and beans from a supermarket.