The oil consumption of a turbocharged, aftercooled direct-injection truck diesel engine was measured using a tritium-tracer technique. The advantages of the method over other chemical and radioactive tracers are described, and supplemented with data from radioanalysis of tritiated oils. As a proportion of fuel consumption, the oil consumption was shown to range from <0.1% to >0.4% depending upon the engine's load and speed, with the highest consumption at idle and at full load conditions. The mass consumption rate ranged from 6 g/h at light load, low speed to 230 g/h at full load, rated speed. The contribution of consumed oil to another truck engine's particulate-bound hydrocarbon emission was shown to be greatest at light and intermediate loads and negligible at high loads. The implications of these observations on diesel particulate control are discussed.