THE PLT WAS ON A TEST FLT AFTER AN ANNUAL INSPN WAS COMPLETED. DRG FLT, AN OIL FILM APPEARED ON THE WINDSHIELD, SO HE TURNED BACK TO LAND AT THE DEPARTURE ARPT (MERRILL FIELD). BEFORE REACHING MERRILL FIELD, THE OIL FILM BECAME HEAVY & OIL PRESSURE BEGAN DROPPING. THE PLT ELECTED TO LAND AT A PVT AIRSTRIP. DRG THE APCH, HE SLIPPED THE ACFT FOR VISIBILITY. HE SAID HE MADE A NORMAL TOUCHDOWN, BUT DRG THE LANDING, THE ACFT DRIFTED TO THE LEFT, HIT A ROCK & SOME HIGH VEGETATION, THEN NOSED DOWN. AN EXAM OF THE ACFT REVEALED THERE WAS A SMALL LEAK IN THE ENG OIL COOLER.