I can't speak for the US fuels, since I'm in Germany.

But in our fuels there are additives allready.
The engines are built for them.

Those additives you can buy and add ar eNOT doing any good to the engine, PLUS the warranty expires, because the engines are not built for it.


But as I said, I can only speak for Germany, where all thise extra additives you can buy have been tested and none of them worked, so all of them are just a rip off.

 

No matter what the additive's manufacturer claims about warranty, I wouldn't use an additive unless your having a specific problem. If some thing goes wrong, part of the conditions of the warranty is that you ship your blown motor (the only evidence that you have) to the additive manufacturer (the ones your trying to get money from) so they can tell you if they will pay for a new engine.
Chevron Techron, for example- Techron states that you should add it just before an oil change because even though its in the fuel it does contaminate the oil, With the extended oil change mileage on our BMW's it has more time to do damage to the engine. Name brand oils and fuels are pretty sophisticated today so you should not need to play chemist yourself on a trial and error basis.