Today, after four perfectly fine flights, as I was putting the model away, I noticed that the muffler seemed very loose and figured that was probably the reason that I was getting a couple hundred less RPMs today. So when I got home I took the cowl off to tighten the muffler and found the engine and cowl covered in "silver slime." The only thing I can imagine is that this is metal filings mixed with the RCV91cd's usual copious oil residue. I cleaned it all off and tomorrow I'll run it again to see if it reappears...but really what is this???

Note: The engine seems to be running very well.