This study examined the effect of adding 50 parts per million [ppm] of either Co₃O₄ or Fe₃O₄ metal-oxide nanoparticles to biodiesel produced from used sunflower oil via transesterification on diesel-engine performance and exhaust pollution reduction. The two powders were first examined using X-ray diffraction, transmission electron microscopy (TEM), and Brunauer-Emmett-Teller (BET) surface area tests. Next, the nanoparticles (NPs) were fed into a four-stroke, single-cylinder engine running at a steady 2500 rpm, and their effect was measured across a range of loads. Measurements included brake-specific fuel consumption (BSFC), brake power (B.P.), brake-thermal efficiency (BTE), brake-specific energy consumption (BSEC), exhaust gases (CO, HC, CO₂, NOₓ, and soot), and noise. Adding either NPs improved BSFC by up to 6.6% and 3.3% for cobalt and iron oxides, respectively, and improved efficiency compared to straight biodiesel by 2.7%, 1.4%, respectively. Co₃O₄ outperformed Fe₃O₄, likely because it carries more oxygen and a higher calorific value. Both additives reduced CO and HC by up to 23% and 18%, respectively, yet increased NOₓ; Fe₃O₄ caused a smaller increase. Overall, metal-oxide NPs show promise as simple, low-dose boosters for cleaner, more efficient diesel engines.