I faced this problem a few months ago. Now solved already. I think mine was more severe, all 5 head cover nuts were covered in oil (my engine is a weak D16Y7 non-vtec ). The problem with most honda head cover is that the thread in the head itself is not through-hole type. Meaning that, if the seal under the nut is weak already, no matter how tight you torque the nut it will still be covered in oil. The solution: replace all the seal and put some new head cover gasket for good measure.
Maybe the thread loose already? If it is, go find workshop that has tap & die set, the thread can be re-threaded. You'll have to use slightly larger nut after that.
So here am I again, bringing my "privilege" of attending the Retro Toyota gathering at Ace Cafe to my fellow ZTH-ians. Together showing the culture JDM do exist even in places...