Nanofluids have recently captured the attention of researchers due to their numerous advantages, including improved cooling, lubricating, and thermal properties compared to pristine fluids. Producing a highly stable vegetable oil-based nanofluid with optimum kinematic viscosity is an important task for lubrication, which plays a crucial role in prolonging the lives of machines. This work investigates the stability, viscosity, and sustainability of castor nanofluid prepared by blending 0.1 wt.-% boehmite nanoparticles (B-NPs) under ultrasonication. The zeta potential of the nanofluid was measured to be −34.7 mV, indicating its good colloidal stability. The kinematic viscosity (KV) of castor oil was 49.81 ± 0.23, 25.38 ± 0.11, 14.77 ± 0.05, and 10.42 ± 0.02 cSt at 40 ± 0.5, 60 ± 0.5, 80 ± 0.5, and 100 ± 0.5 °C, which was enhanced by approximately 27 cSt, 13 cSt, 5 cSt, and 4 cSt, respectively, upon the addition of the B-NPs at the corresponding temperatures. The KV values of both oil and nanofluid decreased with temperature. B-NPs have been demonstrated to be excellent viscosity modifiers, hence attesting to the castor oil-based nanofluid’s potential suitability as a sustainable and eco-friendly lubricant as an alternative to petroleum-based ISO VG 68 hydraulic oil.