:A method of cleaning diesel engine lubricating oil on-line was investigated using a bypass fine particulate filter followed by an infra-red heater to remove water vapour and light diesel fractions in the oil. The impact of this oil recycler on the gaseous and particulate emissions was investigated over a 300 hour oil age period. A Ford 1.8 litre IDI passenger car diesel engine was used with engine out emission sampled every 15-20 hours. The tests were carried out at 2500rpm (52% of the maximum speed) and 12.3 kW with 47 Nm load (43% of the maximum load and 29% of the maximum power). The EGR level at this condition was 15%. A stop start test cycle was used with a cold start each time and a typical test period of 2-3 hours. The results showed that the recycler had its greatest influence on emissions for fresh oil when there was a large reduction in particulate emissions due mainly to large reductions in the ash, carbon and unburned lubricating oil fractions. Bosch smoke emissions were reduced with the recycler and increased when it was removed after 310 hours. The recycler kept the particulate emissions at the initial low level for 310 hours, with oil top-ups at 220 and 290 hours. There was evidence that the addition of fresh oil and the use of the recycler promoted deposit removal from the engine that was followed by a hydrocarbon, CO, NOx and particulate emissions peak. The particulate mass emissions gradually decreased with oil age with the recycler, from 1.7 g/kg to 1.3 g/kg at 310 hours. Without a recycler the particulate emissions were 3.3 g/kg for fresh oil and 2.2 g/kg for 100 hours aged oil. The recycler produced a major reduction of the flow of carbon into the lube oil, from 10-15% of the carbon emitted down to 2% of the carbon emitted with the recycler. This played a major role in extending the life of the oil.