The back spark plug on my 82 FXRS always has what looks like burnt oil on one side only while the other side looks OK. The back head had a valve job before I got it I think because the guides look new compaired to the forward head (wasn't there a recall for the back valve guides at some point?) The discoloring doesn't seem to hurt the performance but I switch the plugs around once in awhile to keep the back one from getting to gummed up. My guess if it was oil past the rings the plug would foul more evenly so I'm thinking a valve guide seal but I just don't know.