As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CRUISE FLIGHT, NOTICED A HIGH MAIN TRANSMISSION TEMPERTURE. AT THAT TIME MY PASSANGER IN THE BACK SEAT INFORMED ME THAT HE COULD DETECT A BURNED OIL ODOR. TURNED THE ACFT AROUND AND RETURNED TO DEPARTURE. MX DRAINED A LITTLE MORE THE A QUART FROM THE TRANSMISSION DUE TO OVER SERVICE. TOOK OFF FOR A MX TEST FLIGHT AND THE PROBLEM WAS RESOLVED.