First, I do use Seafoam and have for awhile. I am not a believer in "snake oils". In fact, Seafoam is the only thing I ever add (other than some Iso-heat to prevent freeze up in our winters). It made a huge difference in my JD 3020 gas which would have carb issues without the seafoam. I had several tractors, and the JD would sit for awhile and didn't always have the freshest gas. My wife had a '98 Isuzu that would throw "check engine" codes pretty regularly. A fill up or two with Seafoam would solve that. And yes, I tried not putting Seafoam in and running it and the code stayed (or, if cleared, came back). I don't have any real numbers, but my '84 pickup seems to run better w/ some Seafoam. Both of these vehicles sit for awhile between uses. I also add it to my 2-cycle fuel (which can also sit awhile).
Note that I'm not using it as a fuel preservative, rather to clean out the gunk and rust that grows in metal tanks and fuel lines as things sit and rot.
I did start making my own (only use in 4-cycle vehicles that are not in warranty--still use the real stuff in the chainsaws). The mix I use is 4:2:1 #2 diesel:Naptha:Iso Heat. At that price point I can afford to dump it into lawn mowers, tillers, etc for the first and last tanks each summer, the two older vehicles and the Gravely walk behind every tank. Again, the chainsaws get real seafoam along with their two cycle oil.
This seems to work for me. YMMV, try at your own risk, etc.
