What was found
TEN MINUTES INTO FLIGHT, LEFT ENGINE OIL PRESSURE BEGAN TO FLUCTUATE AND ALL OTHER INDICATIONS WERE NORMAL. TWENTY MINUTES INTO FLIGHT, LEFT ENGINE OIL PRESSURE DROPPED TO ZERO AND OIL PRESSURE LOW LIGHT AND OTHER INDICATIONS NORMAL. ACFT RETURNED TO MKE. FOUND WIRES BROKEN INSIDE THE INSULATION AT THE OIL PRESSURE TRANSMITTER ELECTRICAL CONNECTOR. REMOVED AND REPLACED THE CONNECTOR, OPS CHECKED GOOD.