Lubricant formulation was found to have dramatic impact on legislated emissions in a study using an ultra low sulphur fuel, a 12 litre naturally aspirated Euro I representative engine and tested over the ECE R49 test cycle. An SAE 10W-40 high quality part-synthetic heavy duty diesel engine oil was found to generate over 20% less particulate matter than two fully synthetic products of higher performance specification (SAE 5W-40 and 10W-40), whereas the fully synthetic oils showed a 10% reduction in Nox. All three oils are commercially available. The part-synthetic and fully synthetic oils also reduced fuel consumption relative to the SAE 15W-40, all mineral oil reference oil (0.4 - 0.9%). Higher oil consumption related to lower lubricant viscosity index improver content in the synthetic products can explain the results although more work is needed. Alternatively, there is some evidence that the dynamic fuel injection timing may be effected by the lubricants viscometric properties. This may only impact engines equipped with mechanical fuel injection systems. A further study on an increased number of lubricants and using a Euro II engine is underway.