ONE Single Dirty/Oily Spark Plug

I recently changed the spark plugs and the wires on my 1993 Buick Lesabre. However, after doing so, the idle sounded rough, so I was forced to take them all off and recheck the gap on all of them and double check the wires were connected in the right order. After the second try, it sounded a lot better. But it has had for some time now, even before I changed the plugs, a slight misfiring sound that I was never quite able to pinpoint. Until I remembered that when I changed the spark plugs, one of them, just one of them, the one on the backside and on the outer edege, was covered in what appeared to be black dirty oil. I just read online that if there is any oil leaking into a spark plug tube, it WILL cause a misfire. I think this is my problem. So my question is, what exactly do I have to replace? I've read that the valve cover gasket is the main suspect, but that if that was the problem, there would be oil leaking all around the
spark plugs on the outside of the block. But this dirty oil is only present on the threaded part of the spark plug. So it's gotta be coming from the inside the plug tube. I still plan on replacing the valve cover gaskets but what else needs to be replaced with that? Thanks.