* Cracked cylinder head: This is a significant failure, often caused by overheating or age-related deterioration. The crack allows oil to seep into the cooling system.
* Blown head gasket: This is the most common cause. The head gasket seals the combustion chambers from the cooling passages and the oil passages. A failure allows oil and coolant to mix. A damaged head gasket can show several symptoms, and oil in the coolant is a major one.
* Cracked engine block: Less common than a cracked head or blown head gasket, a crack in the engine block can also allow oil and coolant to intermix. This is generally a catastrophic failure requiring engine replacement.
* Oil cooler: The oil cooler is part of the cooling system; if it fails internally, it can allow oil to leak into the coolant.
Determining the exact cause requires a thorough inspection. Simply adding coolant won't fix the problem; it'll just continue to mix with the oil, eventually leading to engine damage. Driving the truck with oil in the coolant can cause catastrophic engine failure due to overheating and lack of lubrication.
Do not drive the truck until the problem is diagnosed and repaired by a qualified mechanic. Continuing to drive it risks severe and costly engine damage. A proper diagnosis will involve a pressure test of both the cooling and oil systems, likely requiring the removal of parts to inspect the head gasket and other components.
