The goal of this research is to determine how different types of engine oil affect how well a four-stroke, single-cylinder, air-cooled, direct-injection diesel engine performs and how much pollution it produces. At 1000, 2000, and 2500 rpm, the engine was linked to a hydraulic dynamometer and driven with a continuous load of 4.0 N·m. Local Baghdad Oil (B1), Gulf Oil (G1), and PRO-TEC Oil (P1) – three lubricating oils – were assessed. Using an AIRREX HG-540 gas analyser, engine performance and emissions were assessed based on brake-specific fuel consumption (BSFC), exhaust gas temperature, and nitrogen oxides (NOₓ), carbon monoxide (CO), and carbon dioxide (CO₂) emissions. For all oils, results show that BSFC dropped as engine speed rose. PRO-TEC Oil (P1) had the lowest BSFC of 0.290 kg/kWh at 2500 rpm; Baghdad Oil (B1) had the highest value of 0.351 kg/kWh at 1000 rpm. Exhaust gas temperature increased with speed, reaching a maximum of 243°C for B1 at 2500 rpm; P1 consistently had the lowest exhaust gas temperature, with a minimum of 188°C. As engine speed rose, CO emissions rose; P1 had the highest value at 1000 rpm (0.018%), while B1 had the lowest (0.013%). Rising speed also increased CO₂ and NOₓ emissions; B1 had the highest NOₓ (300.1 ppm) and CO₂ (2.451%), whereas P1 consistently showed the lowest emissions. The outcomes highlight the advantages of oils with improved viscosity and thermal stability for enhanced performance and decreased environmental impact by showing that engine oil qualities greatly influence diesel engine efficiency and emissions.