This paper reports on the first successful tests performed on a production D.I. Diesel engine using wood pyrolysis oil (WPO). As reported in literature, any attempt to directly replace Diesel fuel with WPO required extensive modifications to the engine injection system, in order to overcome the intrinsic limits of the oil (poor self-ignition, high acidity and viscosity): new materials, additional pilot injection systems, careful procedures of start-up and shutdown were needed to obtain acceptable operation. Aim of the present work was to assess the limits of utilization of WPO in a strictly stock engine. Therefore, while no modifications at all were carried out on the engine, the efforts were addressed to make the WPO compatible with light-duty Diesel engines. Several long-running tests were performed on a single-cylinder engine, with: blends of WPO with different percentage of oxygenated compounds micro-emulsions of WPO in Diesel fuel standard (commercial) Diesel fuel. Reliable operation was achieved with percentages as high as 44.1% of WPO in Dyglime (Diethylene glycol dimethyl ether). Similar results were obtained with two different emulsions with 30% of WPO in No.2 Diesel fuel. While residuals were occasionally found to stick on the nozzle stem and sac, no trace of corrosion was detected in the injection system. Detailed analysis of the pressure cycles and emission profiles have been carried out, highlighting the feasibility of an immediate utilization of WPO in D.I. engines.