I've been noticing that there is an excessive amount of grime built up on the top of my engine (i think they're called the cylinder heads?) - around where the spark plugs go in. My guess is that some oil is seaping out from somewhere, but I can't really determine where, unless it could be from around the plugs. Can someone tell me where this is coming from, and what I need to do to fix it?

I'm not sure if it's related, but I've noticed that the performance has been declining a bit in the past few months, despite a few performance mods (PV Muffler, Cone Filter).

Any advice would be appreciated!