While en route to Cincinnati, OH, approximately one hour after takeoff from Maytown, PA, the pilot and passenger observed oil on the airplane's windscreen. The pilot stated he decided to make a precautionary landing, but as soon as he turned towards the alternate airport, he noticed an excessive amount of oil flowing across the airplane's clear floorboard. The pilot said he started to look for suitable terrain to make a precautionary landing, but there was none. At this time, the oil pressure gauge was reading zero. He said he pulled the power back to an idle setting, hoping that if he needed power later, the engine might still be functional. He said he flew the airplane into a small clearing in the wooded terrain. The airplane hit the field, turned 90 degrees in direction, and slid on the right wing for 100 feet, coming to rest approximately 250 feet from a treeline. On-scene investigations revealed that the oil breather line had accumulated ice near the end of the line and was totally blocked. The oil leak originated from the crankshaft oil seal.