Oily air filter

[solanog]

What's going on when the air filter seems damp with oil? I replaced a while back the 1990 Accord air filter and the old one seemed like it was moistened with oil, it seemed that the oil came not from the outside (bottom) but from the engine (top). Why would this happen? the intake hose seems dry on the inside. The guy that changed the filter (at the oil change in the service station) said that it could be the valve seals, is this true? What else could it be? I checked it now(after a couple of thousand miles) and it is dry, dirty but dry