Couple of things to look for. One is your oil cap seal. They get very hard and brittle and can cause oil to leak and seep through down to the plugs. Another place and very expensive to repair would be the spark plug holes seals. If the seals are leaking from there, the camshaft cover needs to be removed to replace the orings and reseal the cam cover. Special tools are required to remove and install the cover and then lock the cams in place to set up the timing again.