Recently changed air filter. Filter and inside of airbox were covered with a thick, black, tar-like substance. Some of the filter element had separated from its backing and was stuck to the box, and the rest of the filter was completely covered with this crap. It is a wonder that the car ran at all.

My hypothesis is that unburned oil is leaving the exhaust, making its way through the preheat tube and into the airbox. Does this sound plausible? If so, what would cause that to happen? Clogged flame trap? Bad ring(s)? Stuck or burnt exhaust valve? (Think that it is safe to rule out the flame trap, as it is only a few months old.)

Motor is a B230F, with approximately 205K miles. Seems to run fine otherwise. Did notice that the oil level on the dipstick dropped suddenly and rapidly shortly before (~1wk) this "discovery."

Is it likely that this "goo" is something other than oil? What else might it be?

Incidentally, the airbox was also found to have two small holes in it. Not sure if this is related, or due to corrosion from battery acid, or something else entirely. Interesting thing is that the holes were about the same size and at almost exactly the same spot, and on exact opposite sides of the airbox (top). Temporarily patched with tape until airbox can be replaced.

Any ideas, comments, suggestions or advice would be greatly appreciated.
TIA!