Have you been putting a different type of oil in,?
ie, synthetic or semi synthetic, when maybe mineral oil was already in,?
If not look inside the coolant header tank to see if there is any yohurt in there, if there is ,suspect the cylinder head gasket,
If no to both just keep an eye on things for awhile till the cold weather has gone, then change the oil.
This used to be a common thing years ago, and was caused by cold air hitting a source of hot oil, although we used to call it sallad cream,
Let me know the result,
Good luck.