I pulled the plugs to change them: oil on the threads, otherwise they look fine. Truck also hesitates and misfires at times. Exhaust stinks like unburnt fuel, as though running super-rich. Easy to start. 32k on engine, 2008 GMC Sierra (truck is 4 years 10 months in my ownership, bought new). 3 years ago, it ran fine, getting close to 22 mpg highway Birmingham to Seattle. Now getting closer to 11 in town, maybe 16 on the highway. Oil changes every 3500 to 5000, manual suggests every 6k.
I have had oil on plug threads before, from spilling while filling, and one truck that had continuously leaky valve cover gaskets, but this appears dry on the outside, so I assume the oil is coming from inside the engine somewhere.