The pilot reported that, as he exited the airport traffic pattern, he noticed that engine oil was accumulating on the left portion of the windscreen, and he decided to make a precautionary landing. During the approach, the left portion of the windscreen was obscured, and the pilot switched from the left to the right seat. The pilot fastened his seat belt, continued the approach, and touched down on a soft spot on the snow-covered turf runway just short of his aim point. The airplane’s nose landing gear dug into the soft surface and the airplane nosed over. Additionally, the pilot reported that during his postflight inspection, he noticed that the oil filler cap was separated from the filler port and dangling by its security chain. The pilot opined that during his preflight inspection, he saw the oil filler cap but did not physically twist the cap to ensure its security. The pilot reported that there were no mechanical malfunctions or failures with the airplane that would have precluded normal operations.