The original Seafoam product can be used different ways - in oil, gas, intake cleaner....
I use it in gas, as a occasional cleaner. Works well, up to a point - if you have a carb that is totally clogged up with dried junk, often nothing except complete disassembly and manual cleaning/replacement of parts will work. But, for carbs that have been somewhat neglected, it can often help quite a bit.
Recently I've heard that some people have created problems by using way too high of a dosage in gas - affecting seals. I have not experienced this, nor been able to verify otherwise, but if it is true, then I view it as a positive thing - means to me that the product is strong enough to be useful. Just follow the dosage ratios correctly, and you shouldn't have an issue.
Rgds, D.