My truck started running rough a couple weeks ago and after checking the plugs I found my #8 plug seriously covered in an oily muck. I replaced it but it only lasted a few days. I pulled the same plug and again it was as nasty as before. I've read all your posts about bad rings, valve stem seals, and blown plenum gaskets and first thing tomorrow I plan on checking all of those. But here is the weird part...This past week we had a real cold spell. All of the sudden the damn truck stops running rough and drives fine. Then yesterday the temp gets back up above freezing and as soon as I start the truck, it's running rough again. If it's the rings or valves or plenum, is this normal.