This white, milky substance is an emulsion formed on the surface of the fluid or an oil-wetted part and indicates the presence of minute amounts of water mixed with oil. It is probably harmless. To be sure, you need to make sure you do not have water in your oil. You can do this by asking your oil analysis lab to test a sample of your oil for water (If you are not analyzing your oil, we can send you a test kit). If the lab indicates you have water in your oil, replace the oil with fresh oil.
If the lab indicates that your oil does NOT contain water, then the emulsion you are seeing is harmless and is the result of condensate in at least this part of the engine.
This kind of phenomenon has been observed in engine oils that contain potent dispersants and rust inhibitors, as present in Exxon Aviation Oil Elite 20W-50 engine oil. Condensation can occur even when an engine is stored in a warehouse, or where humidity is relatively low. One of the products of combustion is water vapor. Hot air around the engine may be saturated with water. As the engine cools, water can condense onto cool surfaces. While any water in the engine has probably boiled off, small amounts of condensation may collect around the fill cap and mix with oil droplets to form a creamy emulsion.
Exxon Aviation Oil Elite 20W-50 oil will do its job in protecting against rust from this type of moisture.