Pollutant emissions such as carbon monoxide (CO), hydrocarbons (HCs), nitrogen oxides (NOx), and particulate matter (PM) from diesel engines have serious adverse effects on both human health and the environment. Advanced post-engine emission control systems, such as the diesel oxidation catalyst (DOC) and selective catalytic reduction (SCR), have proven effective in substantially reducing or minimizing emissions of CO, HC, and NOx. Additionally, the use of metal-based fuel additives in diesel fuel has been widely studied and applied in practice to improve engine performance and optimize emission outcomes. The interaction between metal-based fuel additives and the performance of DOC and SCR systems has become a key area of research focus. This study investigates the impact of metal-based fuel additives—including cerium (IV) oxide, copper (II) oxide, magnesium oxide, nickel (II) oxide, and titanium (IV) oxide—on the performance of DOC and SCR catalysts under various engine load conditions. In the experiments, conventional DOC and SCR catalysts were used, specifically Pt/Al2O3 for the DOC and V2O5-WO3/TiO2 versus Ag/Al2O3 for the SCR. The variations in CO, NO, and NOx levels in the exhaust gas were monitored, and the efficiency of the catalysts in converting these emissions was calculated and analyzed. The results indicate that the combination of metal-based fuel additives with post-engine emission control technologies can effectively reduce pollutant emissions from diesel engines. Among the metal-based additives tested, cerium (IV) oxide and nickel (II) oxide were found to be particularly effective in enhancing the conversion efficiencies of DOC and SCR systems.