I had this happen too, it's pretty much guaranteed to be your valve-cover gasket. I recommend replacing your gasket with a pre-formed fel-pro gasket.
Make sure you are careful when scraping away the old gasket material, the aluminum head is easily scratched!
You will probably need to use scissors to separate the inner-part(part that keeps oil out of your spark plugs) from the outer-part (Part that keeps oil from spilling out your engine).
Also, I recommend replacing the spark plugs that have oil on top of them. In my experience the oil pooled on top of the plugs will ALWAYS find its way into the cylinder and foul spark points.