We get some complicated questions in the fuel treatment industry – how do PAO detergents work? Why does diesel fuel gel in the winter? Are thiocyanate biocides more effective at killing microbes in biodiesel than carbamates?
On a simple level, gas treatments work when they are present in the fuel before combustion. The recommendation would be to add any gas treatment you want to use during your fill-up, before you add more gas. Administer the recommended amount of gas treatment into your partially-filled tank, then fill up as normal.
This is a “splash blending” procedure that ensures that the gas treatment is properly dispersed throughout the fuel in your tank. And it’s a more effective method than simply putting gas treatment on top of the gas in your tank and leaving it. This would be a “diffusion” method that would eventually see the treatment spread throughout the fuel in the tank (especially given any movement of the fuel while the vehicle is in operation). But the diffusion method isn’t as preferable as adding the gas treatment during your fill-up at the station.
For this question, let’s assume we’re talking about a prototypical multifunction gas treatment like Ethanol Defense or Mix-I-Go. They contain combustion improver and detergents to clean the engine, as well as water controllers for the fuel. The bottom line here is that they’re not going to work if they’re not present in the fuel. So the recommendation would be that, yes, it would be best if you used gas treatment in every tank. Not only will it clean the existing deposits and control any existing water in the fuel, it will also keep the engine clean as time goes on.