A Cessna 140 nosed over during the landing roll after a precautionary landing to a paved access road. The pilot indicated that the oil pressure and oil temperature began to drop for unknown reasons while in cruise flight. The pilot stated he was unable to determine the wind direction and decided to land to the southwest on the road. According to the pilot, the aircraft encountered a right rear quartering 'crosswind' during the landing roll. The pilot estimated the winds to be from the north-northeast at 10 knots gusting to 15 knots. The aircraft weathervaned into the wind and nosed over in the soft shoulder of the access road. The aircraft was examined and it was determined that oil was leaking based on the oil-covered belly of the aircraft. The engine was subsequently examined and it was noted that there was no external damage to the engine that would have caused an oil leak and that the interior of the engine cowling was dry. It was also noted that the engine was equipped with a wobble type quick drain of unknown vintage.