When should engine oil be changed in a diesel engine?

Changing engine oil at 3-5% soot loading is critical for maintaining the performance and longevity of a diesel engine. Diesel engines tend to produce more soot than gasoline engines due to the nature of diesel combustion. As soot accumulates in the oil, it can lead to increased wear and tear on engine components, reduced lubrication efficiency, and potential engine damage if not addressed.

Maintaining oil quality is essential not only for adequate lubrication but also for preventing the formation of sludge and deposits that can negatively affect engine performance. A soot loading of 3-5% is an indicator that the oil has reached a point where it can no longer effectively handle contaminants, thus warranting a change to ensure optimal engine health and efficiency.

Other options, such as changing the oil every 1,000 miles or every six months, don't take into account the actual condition of the oil or engine. It's possible to drive much longer than that based on actual soot levels and driving conditions, whereas "when oil is burnt" is an unreliable indicator since burnt oil can suggest severe engine problems rather than routine maintenance.