Gas engine powered plane do need the odd wipe down now and then but it's usually not bad and not much beyond a bunch of little spots. I'd agree that you are probably a bit rich on one or both of the needles. Also petroleum based oils are more prone to this as well such as Lawnboy, Penzoil at 32:1 versus 50:1 and a few others.

Most synthetics will leave a little less residue.

Before you play with the needles, pull the plug out and try to either describe what it looks like or post a clear picture of it. Do this right after flying it, not just running it on the ground

On a well tuned engine, the ceramic part of the plug will be a medium to light colored tan, any darker then this and you are rich, lighter and you are lean. I'm betting on the plug being a very dark tan to black which is rich.

I would never play with the needles until I read what the plug has to tell me.