I can't add much to the discussion on probable causes, but I've got a different opinion of SeaFoam.

Let me start by saying that it's a FUEL ADDITIVE. I've never dumped any into the oil and have no intentions of it. If your vehicle is properly maintained with regular oil changes you shouldn't have a deposit buildup in the crankcase, head assemblies, valve trains, etc. If in doubt about the crankcase deposits, use an oil additive that's engineered as a high detergent oil additive, like the Rislone oil treatment.

I've used Seafoam in several vehicles as a fuel additive. Some fuels (like Chevron and Shell) have an effective cleaning agent in them that keeps the injectors clean. They will even slowly clean dirty injectors. But many of the discount fuels have minimal detergents in them. They may not foul your injectors, but they can result in a slow deposit buildup that slowly deteriorates the injector performance. If deposits are preventing your injectors from working at their best, Seafoam (or similar additives) can help clean the injectors and restore performance to you vehicle.

Seafoam isn't a panacea. But it can help make an engine run better by cleaning up some of the things that dirty or low-detergent fuels leave behind. This includes moisture which has an opportunity to get into the system every time you remove the gas cap.