My best luck in tracing oil leaks on aircraft engines
has been to clean the engine with Stoddard solvent, dry
it with compressessed air, or otherwise let it dry.

Wait until night and pull the aircraft outside to where
it is nice and dark. Using a Black Light (uV lamp), inspect the
engine. Oil fluoresces, so note any oil residue not removed by the
cleaning, or which may have seeped out while the engine was being
dryed.

Run the engine at slightly above idle for one minute. Keep the
draft from the prop at a minimum while doing so as not
to spread the oil around much. Shut it down. Use the Black Light lamp
to check for new fluorescence.

Leaks that occur only while oil pressure is present should show up
during the first short run. If you dont see a leak, then run the
engine for a few more minutes, and look again. Oil leaks that are a
consequence of splash lubrication might require running the engine
longer or at higher rpms.

This method is particulary helpful in spotting leaks around
through-bolts, cyl base seals, crankshaft seal, accessories on their
mounting pads, etc.

MikeM