WHILE CRUISING AT 8,500 FT THE PAX IN THE RIGHT SEAT NOTICED THAT THE OIL FILLER ACCESS DOOR ON TOP OF THE RIGHT ENG WASOPEN & OIL WAS LEAKING. THE PLT CHECKED THE ENG INSTRUMENTS & THE ONLY ABNORMAL READING WAS A LOW MANIFOLD PRESSURE READING ON THE LEFT ENG. THE PLT ELECTED TO MAKE A PRECAUTIONARY LANDING AT CLINTON. THE PLT STATED THAT WHILE IN THE TRAFFIC PATTERN HE HEARD THE LEFT ENG CUTTING OUT. REMEMBERING THE EARLIER INDICATION O LOW MP ON THE LEFT ENG, HE THOUGHT THE LEFT ENG WAS FAILING & FEATHERED THE LEFT PROP. HE RETRACTED GEAR & FLAPS, BUT THE RIGHT ENG WOULD NOT PRODUCE ENOUGH POWER & THE ACFT CRASH LANDED IN A WHEAT FIELD. EXAM OF THE RIGHT ENG REVEALED A FATIGUE FAILURE OF THE #4 CYL. INITIATION HAD OCCURRED AT PRE-EXISTING SHARP-BOTTOMED DENTS IN THE TWO CRACKED FINS WITH PROPAGATION TOWARD & INTO THE CYL. THE LEFT ENG WAS RUN & THE ONLY DEFICIENCY NOTED WAS THAT THE ENG RAN RICHER THAN NORMAL AT LOW POWER SETTINGS.