The development of sustainable, high-performance lubricant additives is crucial for advancing energy efficiency and prolonging the operational lifespan of mechanical systems. In this study, carbon nano-onions (CNOs) synthesized from plastic waste were evaluated as multifunctional additives in two distinct base oils: mineral oil (MO) and synthetic oil (SO). Rheological measurements revealed that CNOs increased the viscosity of MO, while slightly reducing that of SO, highlighting base oil–dependent interactions. Tribological tests of steel-on-steel demonstrated substantial reductions in friction and wear with CNOs. addition. In MO, the coefficient of friction decreased by up to 46%, and wear rates dropped by 35% at higher concentrations. In SO, friction was reduced by 31% and wear by up to 53% under lower loads. These results position CNOs as an environmentally friendly and effective additive for diverse lubrication systems.