Get in there with a dental pick and scrape the gunk out of the plug, spread it out on a piece of paper, and let it dry. If it's dry and crumbly, it's carbon residue. If it's gummy or putty like, it's
oil residue.
A leaky carburetor can also favor a single hole...a dribble line.
But, yes...it does sound like
oil given the symptoms. You'll probably have to dig into the guides and seals before it's over. I'm still hoping for something easier.
Fiber optic borescopes have gotten pretty cheap these days...that might also be worthwhile before you get too deep. Try Ebay using "endoscope" as a search term.