As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LEFT MASTER CAUTION ILLUMINATED ON TAKEOFF BUT QUICKLY EXTINGUISHED. DURING EN ROUTE CLIMB THE LEFT ENGINE OIL PRESSURE STARTED TO FLUCTUATE 40-50 PSI INTO THE YELLOW ARC. CAPTAIN ELECTED TO RETURN TO FIELD AS A PRECAUTION. CAPTAIN INFORMED DISPATCH THROUGH ARINC PHONE PATCH. MX REPLACED THE AXIS BB PLAIN SEAL.