Lycoming's website has a ton of good info. Here's an excerpt that may answer your question:
"Particularly during cold weather operation low power operation may allow both the oil and cylinder head temperatures to fall below the normal range. This is detrimental to good engine health. Oil temperature in particular should be maintained between 165oF and 220oF to achieve maximum service life. At lower temperatures, the moisture which gathers as a result of combustion will not vaporize and be expelled. This can cause dilution of the oil which detracts from its lubricating properties."
More at http://www.lycoming.textron.com/main.jsp?bodyPage=/support/index.html
