I use an additive all the time in my outboard fuel..... Pour gallons of the stuff into the tank in a year....... It's got a funny formula sounding name......
TCW3!
Joking aside my personal experience is that a lot of these things seem to be a bit of a placebo. Only one I ever used which actually did something was a diesel cleaner you sprayed in with the engine running. Worked by making it backfire and the resulting pressure pulses shook the soot out the EGR valve.
It did appear to work. (and apologies if you were the car behind me that got covered in 200,000 miles of accumulated EGR soot as I drove off after the application....