I would pull the valve cover off and spin it over slowly by hand. See if you see anything strange happening near the valve in question. That oil mixture has to be coming from somewhere. i think it would take a really worn out valve guide if it wasn't a head gasket or crack. I had a minor crack in the head on my G54B. It would only act up at sustained high speed (75mph plus) or sitting in a lot of traffic, both only on a hot day.