As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CRUISE FLIGHT AT NIGHT, THE WINDSCREEN BECAME COVERED IN OIL. THE PILOT MADE AN EMERGENCY RETURN TO THE AIRPORT AND MADE AN UNEVENTFUL LANDING.. INSPECTION REVEALED THAT THE EXPANSION PLUG IN THE CRANKSHAFT (LYCOMING P/N-STD-1211) HAD BEEN DISPLACED ALLOWING THE OIL TO LEAK.