Oil that has a milky, clouded appearance is likely contaminated with?

The milky, clouded appearance of oil is most indicative of contamination with engine coolant. This is often a result of a failure in the head gasket, a crack in the engine block, or a damaged intake manifold, which allows coolant to mix with the engine oil. The presence of coolant in the oil disrupts its characteristics, leading to the formation of emulsified substances that give the oil a cloudy or milky appearance.

In contrast, other contaminants such as fuel or water may present differently; fuel contamination typically leads to a thinning of the oil and may have a distinct odor, while water contamination, although it could also lead to a cloudy appearance when emulsified, often does not have the same milky texture associated with coolant. Dust, while a contaminant, does not usually cause a visible change in the oil's appearance to this degree. Therefore, the appearance of oil as milky or clouded is most correctly associated with contamination by engine coolant.