If your vehicle is showing engine oil on the dipstick looks like a brown milkshake, here are the most likely causes and the parts you'll need to fix each one.
The cylinder head gasket seals the engine block to the cylinder head. When it fails, pressurized engine coolant can pump directly into the oil passages, violently mixing into a brown milkshake.
Many modern vehicles use a liquid-cooled oil cooler. If the internal seals rupture, or if the O-rings are pinched during a recent installation, high-pressure oil and coolant will intermix.
On many V6 and V8 engines, coolant flows through the intake manifold. A degraded gasket, or improper RTV sealant application during recent engine work, allows coolant to leak down into the engine valley and mix with the oil.
On certain engine designs (like the Ford 3.5L V6), the water pump is located inside the engine behind the timing cover. When its internal shaft seal fails, it dumps coolant directly into the oil pan.
Severe overheating or freezing (due to a lack of antifreeze) can physically crack the metal of the engine block or cylinder head, creating a permanent, catastrophic path for coolant to enter the oil.