This study aimed at improving the performance of engine oil (SAE50) by adding zinc oxide nanoparticles. First, the microstructure of nanoparticles was studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). After confirming the microstructure of nanoparticles, a two-step method was used to prepare the engine oil/zinc oxide nanolubricant. The nanolubricant remained stable without any sedimentation for more than 5 weeks in staticmode. Different volumefractions of the nanolubricant were prepared and the rheological behavior and nanofluid viscosity were studied in the range of 25 to 65 °C and, additionally, the cloud, freezing and flash points were measured. According to the results of dynamic light scattering (DLS) test, the nanoparticles in the base nanofluid had an average diameter of 55 nm. The viscosity increased with increasing concentration up to 25.3% relative to the base fluid. The viscosity of the nanolubricant decreasedwith increasing temperature. An accurate relation was proposed based on different temperatures and nanofluid volume fractions to calculate the nanofluid viscosity. The cloud, freezing and flash points were improved respectively by 22.2, 19.4 and 7.2% at the highest concentration, indicating the reasonable performance of nanoparticles.
