Sludge inside oil cap

If there's no other symptoms, including loss of coolant, that is caused by condensed moisture build-up from excessive short-trip driving. The engine is not getting up to normal temperature to allow that moisture to vaporize and be pulled out by the PCV system. A good 30-mile run once a week at highway speed will eliminate most of that moisture.

Have the cooling system tested with the block test system as well as a pressure test. I have a 2003 Expedition that my wife put puts here and there and has never looked like that, but not saying that's impossible though. Looks like a classic milkshake head gasket failure.