Check and see that the slit in the oil drain tube
is not plogged up inside the cowled area. If the slit is
plugged with old oil gunk, the tube will suck oil overboard
and since they stick out a bit into the airstream, you probably
won't see a bunch on the belly. ANOTHER PLACE is a clogged
air/oil seperator on the vacuum pump if a wet pump is installed.
They can pump much oil overboard and still function.