Aiming to solve the problem of the gear lubrication of unmanned aerial vehicles (UVAs), a grease mixed Thermal-elastohydrodynamic lubrication (TEHL) model is established, and the lubrication state of the gearbox of oil-powered heavy-duty UAVs is numerically analyzed. In this study, grease is regarded as a non-Newtonian fluid whose properties change nonlinearly with respect to pressure, temperature, and other conditions. The complex meshing state of a spiral bevel gear tooth surface is analyzed using the loaded tooth contact analysis (LTCA) method, and three points on the tooth surface are selected as samples to describe the lubrication state of the entire tooth surface. The thickness of grease film and the temperature rise of the tooth surface are studied in detail, considering the natural roughness of the tooth surface and influence of different greases. The results show that the established model agrees with results from the literature. The dangerous point of tooth surface lubrication is at the meshing midpoint. Under cruise conditions, the lubricating grease viscosity significantly influences the temperature rise of the tooth surface.