Sounds like condensation in the crankcase. Plowing snow is not normally (in my experience) hard work for a Cub. Given you're working in cold weather and not working the engine hard, the engine does not get up to a real warm operating temperature, a temperature hot enough to drive off the moisture that has collected in the crankcase. After you shut down the engine, the engine cools and draws moisture laden air into the crankcase, through the breather which is perfectly normal. The next time you start the engine to plow snow you have more moisture in the crankcase than the last time you ran it. The cycle continues until:
the weather warms or,
you change oil or,
you get the engine hot enough to drive off the moisture through the breather (the same way the moisture got in).
The solution, for cold weather snow plowing, may be a small amount of covering over the radiator to increase engine operating temperature to effectively drive off the accumulated moisture.
Hope this helps.
Boy! You guys are quick!
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