On November 29, 1998, about 1800 Atlantic standard time, a homebuilt Quicksilver MXL-2, N6153E, registered to a private individual, operating as a 14 CFR Part 91 personal flight, crashed into trees short of Humacao Municipal Airport, Puerto Rico. Visual meteorological conditions prevailed and no flight plan was filed. The airplane received substantial damage and the recreational pilot and a passenger sustained minor injuries. The flight originated from the same airport about 2 hours before the accident. According to the pilot, he had performed a gradual descent from 1,500 feet agl, to 500 feet agl, in preparation for entering the landing pattern when the engine quit, unexpectedly. All engine instrument indications were normal. The pilot was about 2 miles from the airport, and his only option appeared to be an emergency landing in a housing area. The pilot was able to land on a road, and stated that he, "could not get down fast enough even though slipping and diving. We touched down just at the end of the road and entered a yard with trees...". Collision with the two trees caused the damage and the injuries. The pilot stated he had 11 gallons of fuel on takeoff. The fuel consumption of the Rotax 503 is about 3.5 gallons per hour at normal cruise. Subsequent disassembly inspection of the two cylinder, two cycle engine, with FAA oversight, revealed that oil coking occurred on both pistons between and below the piston rings and metal transfer occurred between the upper half of the piston skirt and cylinder wall on the aft cylinder. Both sets of piston rings were stuck within their individual ring lands. According to the FAA inspector, the owner/operator was doing his own premixing of automotive gas and a synthetic oil using a mixture ratio of 100:1. According to Rotax factory personnel, synthetic oils are not recommended for premixing in their engines because of its high hydroscopic properties, especially in high humidity areas such as San Juan. Additionally, Rotax factory personnel stated a synthetic oil does not "coat" between last shutdown and first startup as well as mineral based oil.