I changed the spark plugs for the first time on my 2008 Mazda3, ~118k miles. Three of the four plugs had oil on the threads, on the top of the plug (where the wrench sits), and on the electrode. The only clean plug was the one on the far passenger side.
Some research I did indicated that this could be due to bad gaskets, valves, O-rings, pistons - or possibly just a sloppy oil change. I just had the oil changed at a local shop a few weeks prior. How can I (as an amateur DIYer, if that wasn't obvious!) determine what caused oil to get on the spark plugs?
As far as I can tell, there's no blue or oily exhaust, and the engine idles and runs OK.