I like the idea of taking the oil cap off, but it might depend upon engine make, as some fillers are on the end of a long tube. Also the engine breather should be doing that sort of job anyway. I used to fly a c150 that some weeks had milky oil and other weeks had clean oil, never got to find out why... The link to the water ingestion video, that shows water can get into the oil from the carburettor...
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