Nanocomposites have attracted significant attention as lubricant additives due to their advantages in reducing friction, enhancing wear resistance, and improving thermal and oxidative stability. In recent years, increasing research has explored how different types of nanomaterials (such as carbon-based materials, metallic nanoparticles, and ceramic phases) can use synergistic effects to achieve performance surpassing that of their single components. This review focuses on relevant studies published between 2020 and 2025, providing an updated overview of the advantages, synthesis methods, structures, dispersion stability, lubrication mechanisms, and tribological behavior of nanocomposites. Various structural types are discussed, including core–shell, layered, and in situ hybrid systems, along with their fabrication routes, such as sol–gel processing, hydrothermal synthesis, and surface modification strategies. The lubrication mechanism of nanocomposites is analyzed based on the material structure and the testing conditions. Particular attention is paid to the synergistic effects among multiple components within the nanocomposites and to how these synergies enhance tribological performance. Furthermore, the challenges faced by nanocomposites and potential future developments are discussed. This review aims to clarify the current status of nanocomposites as lubricant additives and facilitate their future application in advanced lubrication systems.