Personal opinion - I never suggest that additives are used. Have they been tested on all cars to be safe? Probably not.
For peace of mind, I prefer taking my car to the dealership and having them properly flush the systems vs. adding compounds to the existing structure which do not truely clean them out. Yes it costs more, but what you want is to save that money in the long-run by having it done properly.
I'm reminded of this shower cleaning stuff I bought. After each shower, you spray this stuff on the wall to keep calcium deposits from forming. It works fairly well, but it doesn't ever really clean the shower. Same goes for fuel and other additives for cars.
Hell, my dealer is pushing this crap... oil treatments, fuel injection cleaners, etc. None of it does any good as long as you're using decent quality gas and oil, and it really lowers my opinion of the dealer service dept. even further!
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