The polyaniline (PANi)-coated nanodiamond (ND) composites were fabricated by the in situ polymerization process and dispersed in base oil as nanolubricant additives by ultrasonic-assisted dispersion. The morphology and microstructures of the nanocomposites were characterized. The in-house developed reciprocating tribo-device was utilized to conduct the tribological tests. An actual CrN piston ring–nitriding cylinder liner friction pair used in the diesel engine was employed to evaluate the effectiveness of the developed nanolubricant additives. The wear tests were conducted under conditions that were close to the actual work condition of the selected friction pair. Furthermore, the anti-scuffing performance was also investigated and the associated mechanisms were analyzed. The results show the polymerization process inhibited the agglomeration of the NDs that were well dispersed in the PANi matrix. The stable hydrogen bonding interactions and the surface confinement effect promote the dispersion of the nanocomposites in organic base oil effectively. The developed additive can improve the friction and wear performance of the ring–cylinder liner friction pair by 12–19 and 15–24%, respectively, compared with the base oil. With the increase of temperature, the effectiveness of the nanolubricant additives is enhanced. Under the oil-starved condition, the friction pairs lubricated with PANi/ND lubricants can sustain a longer stable period with a lower friction force, and the anti-scuffing time is almost three times longer. Concerning the overall tribological performance, the optimal content of the PANi/ND additive in base oil is 2 wt%.