An experimental study is conducted to evaluate and compare the use of a diesel fuel supplement, specifically a 25/75% and a 50/50% blend of waste olive oil and commercial diesel fuel, in a four-stroke, DI (Direct Injection) diesel engine and in a four-stroke, IDI (Indirect Injection) diesel engine having a swirl-combustion chamber. The influence of the blends (diesel fuel + olive oil), for a large range of loads, has been examined on fuel consumption, maximum pressure, exhaust temperature, exhaust smokiness and exhaust-gas emissions such as nitrogen oxides (NOx), hydrocarbons (HC) and carbon monoxide (CO). The differences in the measured performance and exhaust-emission parameters, from the baseline operation of either engine, are determined and compared. The study shows, for both the DI and IDI engines, a small penalty in specific fuel consumption, a moderate increase in exhaust smokiness and essentially unaltered maximum pressures and exhaust temperatures when using the blends. Also, for both the IDI and DI engines when using the blends, the study shows moderate decreases in emitted nitrogen oxides and increases in hydrocarbons as well as negligible increases in emitted carbon monoxide. Theoretical aspects of diesel engine combustion are used to aid the interpretation of the observed engines' behaviour.