The tribochemical behaviour and thermal stability of graphene nanoplatelets (GNPs) as boundary-lubricating additives in engine oil were investigated through steel-on-steel sliding tests from 25 °C to 120 °C. At 25 °C, GNPs had negligible effect on coefficient of friction, COF: 0.083 in oil-only vs. 0.081 in oil + GNPs). At 50 °C and 80 °C, the COF decreased significantly with GNP addition—from 0.106 to 0.077 (~27 %) and from 0.114 to 0.088 (~22 %), respectively. At 120 °C, only a modest reduction was observed (0.139 to 0.123), indicating a decline in additive effectiveness at elevated temperature. X-ray photoelectron spectroscopy (XPS) and Raman analysis showed that the tribolayer remained chemically stable up to ~80 °C, with predominant C–C/C–H bonding and limited structural disorder. At and above 80 °C, oxidation became evident, with a shift to O–C=O species and increased D and D′ band intensities. HR-TEM revealed bending and fragmentation of graphene layers embedded within an Fe₂O₃-rich tribofilm, with interlayer spacings increasing to 0.34–0.36 nm. These results indicate that GNPs reduce friction and wear by forming a carbon-rich tribolayer, but oxidation and structural degradation beginning near 80 °C limit their stability at higher temperatures. The findings provide insight into the temperature-dependent structural evolution of graphene under boundary-lubricated conditions and suggest the need for stabilization strategies to extend its high-temperature tribological performance.