As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LEFT ENGINE POWER HAD 3% & RPM(100RPM) FLUCTUATION. VISIBLE ENGINE OIL LEAKING OUT OF AIR OUTLETS. DIVERTED TO XXXX ON THE GROUND, POWER 10%FLUCTUATION RPM. INVESTIGATION REVEALED THAT THE #3 PISTON WAS CRACKED AND A HOLE HAD DEVELOPED AT THE 3 O'CLOCK POSITION. THE ENGINE WAS REMOVED AND A NEW ENGINE WAS INSTALLED.