Which of the following is a sign of contamination in the oil?

• A. Oil is clear and odorless
• B. Oil appears milky and foamy
• C. Discolored oil and acidic smell
• D. Oil is red and has a sweet odor

Answer: (C)

When oil is contaminated, you’ll often see visible changes in color and a noticeable change in smell, because contaminants and chemical reactions are altering the oil’s properties. Discoloration shows that substances have entered or reacted with the oil, and an acidic odor points to acid formation from oxidation or additive breakdown. Together, these signs indicate the oil is degraded and no longer performing properly.

Clean, clear oil without odor is what you want to see in a healthy system. Milky and foamy oil suggests water or air contamination rather than just a chemical change, so it signals a different contamination issue. Red oil with a sweet odor isn’t a typical sign of standard contamination and could be due to unusual dyes or additives, not the common contamination indicators.