AN ANNUAL INSPN HAD JUST BEEN COMPLETED (4/22/85) & EACH ENG HAD ABOUT 1 HR OF FLT TIME. THE PLT STATED THAT HE TOOK OFF & CLIMBED TO CRUISE AT 9000 FT, THEN CLOSED THE COWL FLAPS & SET THE FUEL SETTINGS FOR 75% POWER. WHILE EN ROUTE, HE NOTED OIL TRAILING FROM THE L ENG OIL ACCESS DOOR, BUT THE L OIL PRESSURE WAS IN THE NORMAL RANGE. HE THROTTLED BACK THE LEFT ENG & REQUESTED VECTORS TO THE NEAREST ARPT. THE PLT RPRTD THAT WHILE DIVERTING, HE SAW FIRE COMING FROM THE AFT END OF THE R ENG COMPARTMENT. THE PLT SHUT DOWN THE R ENG, DECLARED AN EMERG & SUBSEQUENTLY MADE AN EMERG/GEAR UP LANDING IN A WET FIELD. DRG AN APCH OVER TREES TO LAND, HE HAD TO MANEUVER TO AVOID POWER LINES. TEARDOWNS OF BOTH ENGS REVEALED MELTING ON THE EDGE & TOP OF SVRL PISTONS. ONE ENG HAD MELTING IN THE HEAD OF 1 CYLINDER & ITS EXHAUST VLV SEATWAS DAMAGED. NO MAINTENANCE OR PRE-ACDNT DISCREPANCIES WERE FND THAT WOULD HAVE RESULTED IN PRE-IGNITION OR DETONATION. THERE WAS EVIDENCE THE RPRTD FIRE WAS ACTUALLY OIL BEING USED & IGNITED BY THE TURBO-CHARGERS.