If your vehicle is showing milky, frothy substance resembling a milkshake on the engine oil dipstick or oil cap, here are the most likely causes and the parts you'll need to fix each one.
Frequent short trips, especially in cold weather, prevent the engine from reaching full operating temperature. This allows normal moisture condensation to accumulate and mix with oil vapor at the top of the engine, forming a milky residue under the oil cap.
The Positive Crankcase Ventilation (PCV) system is designed to draw moisture and blow-by gases out of the engine. If the valve or hoses are clogged, moisture becomes trapped inside the crankcase and mixes with the oil to create a frothy buildup.
A catastrophic failure of the cylinder head gasket allows pressurized engine coolant to breach the oil passages. This mixes large amounts of coolant directly into the engine oil, turning the entire oil supply into a milky milkshake-like substance.
If the cylinder head was recently removed and reinstalled without properly cleaning the mating surfaces, or if the head bolts were not torqued in the correct sequence and specification, the new gasket will fail to seal, allowing coolant to leak into the oil.
On certain engine designs (particularly older V6 and V8 engines), coolant flows through the intake manifold. A degraded intake manifold gasket can rupture and dump coolant directly into the engine valley and oil pan.
Vehicles equipped with an oil-to-coolant heat exchanger can experience internal ruptures. Because oil pressure is higher than coolant pressure, this usually pushes oil into the coolant, but when the engine is off, residual coolant pressure can force coolant into the oil.