There's a real good additive. It's in the gas you buy. Some gas companies advertise it but all have something. I have heard of people getting good results with Sea Foam but you have to wonder what caused the need for it. I have over 400,000 miles on my '88 Grand Caravan daily driver. I've never used any chemical "mechanic-in-a-can" in the fuel system or any other system. General Motors is the only manufacturer I'm aware of that has injector issues that can benefit from the use of cleaners. By "benefit" I mean "runs better", not "solve the problem".
If you feel the need to add something, use a fuel injector cleaner that you add to a full tank of gas. That is a concentrated version of what's already in the gas and it shouldn't hurt anything. Products like Sea Foam are meant more for solving a problem than preventing one.