Why would my car manual say not to add any kind of fuel additives to my gas tank?

Question

I just bought a 2000 Mazda Protege and was skimming through the manual when I came across that statement. I've been using stuff like STP Fuel treatment, fuel injector cleaner, booster, etc. for years on my truck and it runs fine. Why would they say not to use additives?

Answer 1

go for it better than the gas we get

Answer 2

Because there is no control one what is in the additives.

Generally, they are of no benefit (at best), and many cases cause problems.

Answer 3

Because they are not necessary and could damage the engine if used inappropriately.  There isn't a single vehicle manufacturer that recommends using those types of products.