As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CLIMBING OUT, THE #1 OIL PRESSURE STARTED FLUCTUATING. AS TIME PASSED IT OCCASIONALLY DIPPED INTO THE RED AND THE #1 ENGINE OIL PRESS WARNING LIGHT BEGAN TO FLICKER. SHUTDOWN THE ENGINE, RAN THE CHECKLISTS AND RETURNED TO THE FIELD. ACCOMPLISHED OVERWEIGHT LANDING. LANDING UNEVENTFUL. REMOVED FROM SERVICE. FOUND THE #1 ENGINE STARTER / GENERATOR O-RING SPLINE SHAFT TO BE THE INCORRECT ONE. REPLACING THE #1 ENGINE AND PROPELLER.