WHILE CLIMBING ABT 7 MIN AFTER TAKEOFF, THE PLT NOTED A SIGNIFICANT DROP IN OIL PRESSURE. HE THEN HEARD A NOISE FM THE ENG & DIVERTED BACK TOWARD THE ARPT. HOWEVER, THE ENG BEGAN RUNNING ROUGH, THEN FAILED. SUBSEQUENTLY, THE ACFT IMPACTED TRRN DURING AN EMERG LNDG IN A DRY RIVER WASH & BEGAN BURNING. ONE OCCUPANT WAS FATALLY INJURED DURING THE ACDNT. THE PLT (WITH THERMAL INJURIES) & 1 PSGR SUCCUMBED TO INJURIES A DAY LATER. AN EXAM REVEALED EVIDENCE OF A LOSS OF OIL & OIL EXHAUSTION. THERE WAS EVIDENCE THAT A B-NUT FITTING (ON AN OIL LINE FM THE ENG TO THE RGT OIL COOLER) HAD BACKED OFF THE NIPPLE FITTING ON THE ENG. A HVY COATING OF OIL WAS FND ON THE BELLY OF THE ACFT. INVESTIGATION REVEALED THAT THE PILOT PERFORMED HIS OWN OIL AND OIL FILTER CHANGES. HE HAD CHANGED THE OIL & FILTER 2 DAYS BEFORE THE ACDNT. THE SUBJECT OIL SUPPLY LINE WAS ONE THAT WAS TYPICALLY REMOVED DURING OIL CHANGES TO FACILITATE REMOVAL OF THE OIL FILTER AND TO DRAIN THE OIL COOLER.