What is an indication that engine oil has become contaminated with engine coolant?

The presence of a cloudy appearance reminiscent of chocolate milk is a clear indication that engine oil has become contaminated with engine coolant. This specific visual change occurs due to the emulsification of oil and coolant, which creates a suspension of water and oil. When coolant leaks into the engine oil, it disrupts the oil's consistent appearance, leading to this distinct cloudy mixture.

Other indicators, like a darker color, do not specifically point to coolant contamination since oil can naturally darken from use and oxidation. The greenish color is typically associated with certain types of coolant, but it does not in itself signify that the oil has been contaminated. A reddish color might indicate the presence of specific additives or types of transmission fluid, not coolant contamination. Therefore, the presence of a chocolate milk-like consistency is a direct and recognizable sign of oil contamination with coolant, making it the correct choice.