This paper examines the benefits and problems associated with extended oil drain intervals in diesel truck engines. Performance comparisons were made on two-drain periods in three different engines using both mono and multi grade oils. Three typical customer operations were also monitored. Laboratory evaluations were made on typical and severely stressed used oils. Oil filter life and durability were assessed. The economics of extending oil and filter change intervals are reviewed.Efficient combustion and well-filtered air are essential. Potential operational problems, filter durability and economics suggest 40 000 km as being the maximum change interval even under most optimum conditions.