The demand growth for parts and the continuous need to reduce manufacturing costs have been promoting a continuous growth in steel powder metallurgy. However, these materials have a certain porosity and are more subject to wear. As a result, several techniques have been developed to improve the tribological behaviour of sintered steels, such as surface treatments like plasma nitriding. This work evaluated the synergistic tribological effect between the plasma nitriding treatment and the impregnation of the residual pores of sintered steel (Fe + 0.5 wt.%C) with graphite under dry and low-viscosity (4.8 cSt) oil-lubricated conditions. Under dry conditions, nitrided steel exhibited a high average coefficient of friction (COF) of ~0.36, which was reduced to 0.13 with graphite impregnation. The combination of nitriding and impregnation further reduced the COF to ~0.09, demonstrating a synergistic effect. The wear rate decreased significantly from ~10−3 mm3/Nm for nitrided samples to ~10−6 mm3/N · m for nitrided and impregnated samples. In oil-lubricated tests, the COF of nitrided steel was reduced from 0.13 to 0.09 when graphite was introduced, with wear rates dropping by an order of magnitude from ~10−5 mm3/N · m to ~10−6 mm3/N.m. The results suggested a synergistic positive effect of the solid lubricant impregnation process on nitrided sintered steels, especially due to the formation of a homogeneous carbon-rich tribolayer on the tribopair surfaces.