THE ACFT MADE A FORCED LANDING IN A PLOWED FIELD AFTER THE ENGINE FAILED. THE PLT REPORTED THAT DURING CRUISE FLT HE HEARD A FAINT KNOCK FROM THE ENGINE COMPARTMENT. HE REDUCED POWER AND CALLED FSS. ALMOST IMMEDIATELY THE ENGINE BEGAN TO VIBRATE. THE PLT REDUCED POWER FURTHER AND THE ACFT STARTED TO DESCEND FROM 6500 FT MSL. THE WINDSHIELD BECAME OIL COVERED AND THE COCKPIT SOMEWHAT FILLED WITH SMOKE. THE PLT CIRCLED A LARGE FIELD AND UPON LANDING THE NOSE GEAR AND LEFT MAIN GEAR COLLAPSED. THE ACFT CAUGHT FIRE AND BURNED COMPLETELY. POST ACCIDENT EXAM OF THE ENGINE SHOWED THAT #3 ROD FROZE TO THE CRANKSHAFT. THE LUBRICATING SYSTEM SEEMED TO HAVE BEEN FUNCTIONING IN A NORMAL MANNER. THE LAST 100 INSPECTION WAS ON 7/1/84 AT 4139 HOURS TOTAL ACFT TIME. NO LOGBOOK ENTRIES INDICATED ENGINE MALFUNCTIONS. THE PLT SAID THAT THE OIL TEMPERATURE GAGE INDICATED ALMOST IN THE RED AND THE OIL ALWAYS APPEARED EXTREMELY BLACK (ON PRE-FLT).