The pilot observed engine oil leaking onto the windshield. He dropped the aerial advertising banner he was towing on to a golf course and then performed a precautionary landing on the golf course. During landing rollout the aircraft collided with two trees. Postcrash examination of the engine indicated the front crankshaft seal was partially dislodged and causing the oil leakage. The engine crankcase breather line was found partially blocked with oil sludge and debris. No other evidence of failure or malfunction was noted in the engine.