Whew !! ... Time for some degreaser there !! ... lol ...
On topic, the XB's commonly have the rocker box gaskets spring leaks at some point or another in their lives ... usually it's the rear that goes first on them and it leaks oil all over the fan causing it to short out and throw error codes ... (not to mention making a huge mess on and around the engine itself) ... Since yours is coming from around the rocker box area on that cylinder that would be the first thing I'd point my finger at ... Try cleaning that gunkiness off and seeing where fresh oil comes from first ... Might also look at the grommet around the PCV valve and the breather tube coming off of it just in case it might be clogged up with gunk making it come out around the PCV valve itself ...
And what did you mean by the "fill plug" contributing? ...