If your vehicle is showing milky, frothy sludge visible under the oil filler cap, here are the most likely causes and the parts you'll need to fix each one.
Frequent short trips prevent the engine from reaching full operating temperature, causing normal moisture to condense and mix with oil vapor at the top of the engine.
The Positive Crankcase Ventilation (PCV) valve draws moisture and combustion vapors out of the crankcase. If clogged, moisture accumulates and creates a frothy sludge.
A failed head gasket allows engine coolant to leak into the oil passages, creating a milky, milkshake-like mixture throughout the engine, not just under the cap.
On vehicles equipped with an oil-to-coolant heat exchanger, an internal rupture can force pressurized coolant directly into the engine oil system.
If the head gasket was recently replaced, failing to properly torque the head bolts or reusing stretch-to-yield bolts can cause the new gasket to leak coolant into the oil.