I've used SeaFoam to clean out fuel systems, intakes, and crankcases but only a few times because with proper maintenance you shouldn't need it very often. It only works at cleaning because it's basically 2 solvents, naptha and isopropyl alcohol (blended with a couple other additives), that will dissolve some gummed fuel residues and carbon buildups. If you don't think it works, go grab any greasy/carbonized part you've got and rinse it with naptha and/or isopropyl (91%).
In an engine that is heavily fouled due to neglect (like it's been 50k since the last oil change!
), you're not going to get a cure in a bottle. Using it in the crankcase on something like that and you're likely to break stuff free that will possibly clog oil passages. Using it in the top end of an engine in that situation is a bit safer since it will blow the carbon out of the exhaust system but, if it's at that point, it's better to just give the engine a light rebuild and clean all the parts outside of the engine. That will also give you the opportunity to find other problems that may have arisen due to the neglect.
If its an engine that's been taken care of reasonably well and you're just looking for something to help refresh/maintain it, it's perfectly good for that.
I'm not a big fan of adding
anything to automatic transmissions, however because of the intricate seals and friction surfaces. If a transmission is to the point it needs an additive to work properly, it probably needs rebuilt. Otherwise, just drop the pan, clean it, replace the filter, and fill it up with fresh ATF.
For auto transmissions, I use nothing and just follow regular maintenance. Much in the same way you hear hype about seafoam being the greatest thing since sliced bread, you also hear stories of the negative effects of it cleaning crud that keeps parts lubricated and together. I have a hard time doing things that I really can't measure the success. There just doesn't seem to be any real data one way or the other if additives help or hurt so I keep the dollars in my pocket.
About the only cure in a bottle that I do use is sta-bil because it sits in my overly humid garage for weeks at a time. Even with that, I don't know if I am gaining and do so for peace of mind.
If it's only several weeks your gasoline is sitting, you're good without treating it even in a regular vented gas can. I've used gasoline that has sat upwards of 6 months in a similar environment with no issues.
If you do want to treat it for short term storage, like up to a year, Sta-Bil is good. For storage longer than that, PRI-G is the solution (they also make PRI-D for diesel!).
One of the biggest factors in storing gasoline is what it's stored in. Don't keep it in one of those vented fuel cans if you want it to last. It lets pressure out but moisture in. Use a good quality, sealing fuel can like a NATO 5 gallon steel jerry can or one of the 5 gallon Scepter military cans that doesn't vent.
And if you get ethanol free gasoline it will store longer without treatment since there will be no alcohol to attract moisture.
