The development of eco-friendly lubricants is imperative for minimizing environmental pollution, with additives playing a crucial role in enhancing tribological performance. This study focuses on the design and synthesis of high-performance, environmentally sustainable lubricant formulation of surface-modified graphitic carbon nitride (g-C3N4) as additives with bio-based lubricants. g-C3N4, despite its potential as an advanced additive, suffers from poor dispersion in oil-based systems due to its intrinsic surface characteristics. To overcome this limitation, surface modification of g-C3N4 nanosheets was conducted using octadecyltrichlorosilane (OTCS), yielding OTCS-g-C3N4 with enhanced hydrophobicity and improved dispersibility in oily media. The structural and physicochemical properties of OTCS-g-C3N4 were characterized through Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The modified additive was successfully dispersed in castor oil, achieving a significant reduction in the coefficient of friction (COF), reaching a minimum value of 0.018 at an optimal concentration of 0.3 wt %. Tribological assessments, performed using a four-ball tester in accordance with ASTM D4172-94 standards, demonstrated a 54.23% reduction in average COF and a 60.02% decrease in mean wear volume (MWV) as compared with the castor oil alone. Additionally, the last non-seizure load (LNSL) increased from 981N for pure castor oil to 1236N upon additive incorporation, indicating a substantial enhancement in load-bearing capacity. Thermo-oxidative stability was also improved, with the oxidation onset temperature (OOT) rising from 320 to 339 °C. Hemolysis toxicity evaluation indicates a minimal hemolysis rate of 3%, significantly below acceptable thresholds. This economically viable, environmentally sustainable additive, synthesized via an uncomplicated procedure, when integrated into bio-based castor oil, engenders an advanced lubricant system that is congruent with sustainability objectives.