:Over the last 40 years it has been possible to lengthen recommended passenger car engine oil drain intervals by up to five times, despite the substantial increases in oil stress through continously rising demands on performance and environmental acceptability. Behind this considerable progress lie improvements in engine design and production technology and the development of suitable advanced engine oil formulations.With increasing oil drain intervals comes a growing uncertainty as to exactly when the oil change should best be made: a fixed mileage applicable to all vehicles is preferred for its practicality but the optimum depends on the driving history of individual vehicles. In Europe a 15000 km oil drain interval is now normal. A further extension based on a fixed interval would give an advantage to a minority of customers but could seriously compromise the durability of engines in the overall vehicle population.The main objective in automotive development - both vehicle and oil - is to protect the engine whilst safe-guarding the environment and conserving natural resources. Therefore more effective and flexible systems are needed to monitor the condition of the oil through-out the lifecycle of the vehicle so as to optimise the perhaps conflicting needs of performance, durability and consumption.This paper describes how the combined experience of a vehicle manufacturer and an oil company are being used to explore the measurement of engine oil stress and its implications for improved oil change systems that utilise better the performance quality of the oil.