Very seldom do I tear into my carbs on any equipment I have that some varnish is not present. Varnish left to itself will someday get thick enough to flake off and cause a no go. Seafoam is a preventative measure for me. It can also break away a piece of junk or gunk from a newly aquired second hand engine and cause a no go. This will alert you to the problem that a PO didn't take good care of his engine, and you will be able to take it apart and clean it before you get 40 miles out in a cold rain. I don't think it's necessary in the oil, but I use it in my gas about once a month.  Use it. Don't use it. I don't care.