On May 31, 2004, about 1045 Pacific daylight time, a Stinson 108-2, N9816K, descended into terrain shortly after takeoff from the Spaulding Airport, Susanville, California. The owner/pilot was operating the airplane under the provisions of 14 CFR Part 91. The private pilot and the pilot rated passenger sustained serious injuries; the airplane sustained substantial damage. The personal cross-country flight was originating from Susanville, with the planned destination of Fresno Chandler Executive Airport, Fresno, California. Visual meteorological conditions prevailed, and a flight plan had not been filed. During a telephone interview with a National Transportation Safety Board investigator, a witness (who is also a pilot), reported that after departing, the airplane made a climbing left turn. As the airplane reached about 100 to 200 feet above ground level, it began a turn to the right, which he thought was consistent with the pilot attempting to return back to the airport. The airplane descended into terrain about 300 yards south of the departure end of the runway at a wings level, 20-degree nose down attitude. After completing a visual inspection of the airplane, the witness noted that the airplane's windshield was coated in a layer of oil. Another witnesses reported that prior to impacting terrain, the pilot made a radio transmission stating that he was returning to the airport. He announced that he had mistakenly left the oil filler cap off the engine and, as a result, the windshield had become obstructed with oil. In a written statement, the pilot reported that neither he, nor the passenger, remembered the accident. The passenger could recall that after the pilot performed a short field takeoff, and several seconds into the initial climb out, she noticed small droplets of oil forming at bottom of the windshield. The pilot further reported that several witnesses on the ground observed the airplane departing to the south. Shortly after takeoff, the airplane began a steep right turn. Prior to impacting the ground the airplane's attitude recovered to coordinated, level flight. The pilot thought that the accident could have been prevented if he had remembered to replace the oil cap on the engine while performing his preflight inspection. He reported no preimpact mechanical malfunctions or failures with the airplane.