The average motor oil today comes with a standard mix of detergents, additives, corrosion preventers and a number of other chemicals. All of these elements are expected to help engines work better and longer. However, not all mechanics subscribe to the theory of motor oil containing detergent.
Motor oil detergents were introduced in the 1950s. The concept adjusts for the failings of oil filters by attaching to those particles too small to get caught in filters. Instead the detergent holds the particles in the oil so they don’t deposit on engine parts and cause hot spots. If the oil is used for too long, however, it can’t do its job. Thus, flushing out old oil regularly helps maintain the cleaning process and the benefit of the detergents.