sounds like its coming from somewhere else....
if the leak is not coming from the cap...then its originating below the cap....what else is down there?Valve cover gasket throwing a bit of oil magnified by the fan and spraying on the filter housing?
leaky power steering reservoir lines?
is it definitely oil?...not antifreeze, right? not power steering fluid, right?
I would say use a very bright flashlight and try to see where the leak is coming from....
clean the area,start the car and carefully look and see if you can find it....it should start to seep or drip soon, from what you are saying....
yes, pics will help!