Historically, fuel cost conscious customers have tended to purchase diesel passenger cars. However, with increasing competition from alternative fuels and lean burn and direct injection gasoline fuelled engines, diesel engined vehicles currently face tough challenges from the point of fuel economy and emissions. In gasoline engines, low viscosity friction modified oils have demonstrated their potential for reducing internal engine friction and thus improving fuel economy, without adversely effecting engine durability. These fuel economy improvements have led to the introduction of such a low viscosity friction modified 5W-30 oil as the initial and service fill for the majority of Ford products sold in Europe.The trend towards even lower viscosities continues. To assess the potential benefits and issues of moving to 5W-20 in diesel engines, a short pilot study has been conducted using a Ford 1.8l direct injection diesel engine. Five 5W-20 candidate oils were initially evaluated for fuel economy and emissions benefits using steady state engine dynamometer tests. Part load / speed points and full load curves were run for each oil, and the emissions and fuel consumption observed. The current 5W-30 initial fill and a conventional 10W-30 product were also assessed for comparative purposes. The most promising candidates were further tested in-vehicle using a chassis dynamometer to determine the net fuel economy benefit and show any changes in regulated emissions. To help in understanding the differences in the fuel economy characteristics of the oils, in addition to the viscometric differences, the coefficient of friction of the candidates was also assessed using a reciprocating rig. These characteristics were also combined in an attempt to predict the fuel economy improvement.Overall, the use of 5W-20 gave a potentially worthwhile decrease in fuel consumption and therefore CO2 emissions. However, the improvement in fuel economy would need to be balanced against the various costs required to implement such a low viscosity oil. Regulated emissions did not appear to affected.