there is nothing wrong with your engine... it's the nature of the beast.
the glow engines use a really high oil content... like 20% or there abouts. the oil does not burn. So it spits it out the exhaust and ends up on the airframe. 409 or fantastic cleans it right off . thats why glow engines are called "slimers" it's part of the hobby. i got rid of my glows and went gasoline or electric because i hated the sliming so much. (cost of fuel for em is pretty ridiculous too)
Now i am not knocking glow engines at all., had a couple that i just loved the performance of. I could just not deal with the oil all over the plane.