216/235/261 engine crankcases are oil contamination factories, especially if the rings have some age on them.
Blow-by past the rings. Unburned gas will thin the oil.
Condensation from short trips, preventing the condensation from being boiled off.
Anemic stock crankcase ventilation (compared to PCV setup).
It is probably a good idea to change the oil before you button it up for the winter to remove those contaminants while the engine is in storage.
