One of my sparkplugs is oily

You can do the compression test to see if the rings are alright, fi the compression test is ok, I would think you have a valve guide issue on that cylinder. Thats what happened when my valve guides went out.

A compression test is easy and fairly quick. I'd definitely do one. Remember, you're not looking for any specific number (although most peeps here get around 140-150), but rather you're gonna be looking at the difference between the cylinders. I think it's no more than 10 or 15% difference you want. So if cylinders 1,2,3,4,5,6,7 are 150, and cylinder 8 is 130, that's a problem.