On May 25, 1997, about 1320 hours Pacific daylight time, a Cessna 177, N53079, was destroyed after ditching at sea near Rota Island in the Marianas, following a loss of engine power. The pilot was not injured and the passenger received minor injuries. Visual meteorological conditions prevailed for the personal flight and a VFR flight plan was filed. The flight originated at Saipan en route to Agana, Guam. The aircraft was in cruise flight at 5,000 feet msl, about 15 miles southwest of Rota, when the pilot declared an emergency due to engine failure. The aircraft ditched near Harnom Point, about 1/2 mile southwest of Rota. The aircraft sank shortly after the ditching and both pilots were rescued by a fishing vessel. The aircraft was not recovered. The pilot reported the flight was level at 4,500 feet msl and at 30 minutes into the 130-mile return flight to Guam and 10 to 12 miles south of Rota, the aircraft lost fuel pressure and then engine power. He reported that the aircraft departed Saipan with 30 plus gallons of automotive fuel onboard. According to the manufacturer, the engine was designed for 91/96 grade aviation fuel. There are Federal Aviation Administration (FAA) Supplemental Type Certificates that allow the use of automotive gasoline. The approval establishes approved fuel as: unleaded automotive gasoline, 87 minimum antiknock index per ASTM specification D-439/D-4814. Do not use gasoline containing alcohol. With the approvals are requirements to verify the specifications of the auto gas being used and information about the increased tendency for carburetor icing and vapor generation with its use. The aircraft records were obtained from the FAA in Oklahoma City, Oklahoma. No approval via FAA form 337 for the use of automobile gasoline was found. According to the engine manufacturer's Service Letter No. L199, they do not approve the use of any fuel other than those aviation grades specified in their latest edition of service instruction No. 1070L.