I bought a PA-24-260 with a verrry slow leak from the prop governor.
This has taught me some things about oil leaks.

1) Oil does not go in the obvious direction. Even after you take
into account the general airflow around the engine compartment and
baffling there are local variations that make it hard to see how
oil from point A can get to point B (without touching anything
in between!).

2) Oil can move verrrry slowly. When I got the plane the only external
sign of the leak was a fan on the pilot's side of the windshield (the
governor is at the left front above the oil cooler). A few weeks later
(and many flights a bit of oil would creep out from under the dipstick
access hatch. For a long time I thought that was a second leak, but
even after an oil change it was very dirty oil, and then I realized it
was the oil that didn't sneak out the front of the cowling and instead
slowly ran backwards on the underside until pressure forced it through
the gap (other clues included the oil door itself being clean). If you
wipe the bottom of the cowling off it goes away for a few hours.

3) One oil leak can come out multiple places (see (1) above) and at
different times (see (2) above) and look like multiple leaks.

4) A slow leak that is doing something like oiling the windshield
and irritating you may be mitigated by cleaning off the path from the
leak to the windshield. The oil that actually gets to the windshield
may be actually *leaked* on a different flight, and due to (2) moved
oh so slowly into position to get in your way. In the case of the
governor it seems to leak slowly onto the plate that the prop control
rotates and is blown off from there.

--
Ben Jackson

http://www.ben.com/