If your vehicle is showing milky, frothy residue on the oil dipstick or under the oil cap, here are the most likely causes and the parts you'll need to fix each one.
Frequent short trips prevent the engine oil from reaching full operating temperature. This prevents normal moisture from burning off, causing condensation to mix with oil vapor and form a milky residue, usually isolated to the oil cap.
The Positive Crankcase Ventilation (PCV) system is designed to evacuate moisture and blow-by gases. If the valve or hose is clogged, moisture becomes trapped in the crankcase and creates a frothy sludge.
On many engines (especially V6 and V8 configurations), coolant flows through the intake manifold. A degraded lower intake manifold gasket can allow coolant to leak directly into the engine valley, mixing with the oil.
A ruptured head gasket allows pressurized engine coolant to breach the oil passages. This creates a thick, milkshake-like mixture throughout the entire engine and usually causes overheating and coolant loss.
If the vehicle is equipped with an engine oil cooler that uses engine coolant to regulate temperature, an internal rupture will cross-contaminate the oil and coolant systems.