Of the many impurities commonly found in marine lubricating oil, which of the following CANNOT be removed by a centrifugal purifier at normal operating speeds and temperatures?

Correct Answer: Option A — Diesel fuel oil

Centrifugal purifiers separate impurities based on density differences between the oil and contaminants. Diesel fuel oil is miscible with lubricating oil and possesses a similar density, preventing mechanical separation. Consequently, fuel dilution must be managed through proper engine maintenance and oil analysis rather than onboard centrifugal purification.