The vent hose I think you're referring to is a water drain. If too much water builds up in the air box, you drain it out with the hose.
The crankcase is vented differently. A hose from the cylinder head carries blowby to the intake manifold. See p. 1-34 of the service manual. I suppose if your piston rings were worn very badly that so much oil would enter the intake that some would flow back into the air box and you could get oil in the air box drain hose.