Due to the environmental impact caused by the prolonged usage of petroleum based lubricants has emphasised the need of more eco-friendly bio-lubricants in industrial machineries. For different tribological applications, the bio-lubricants in the original form are not effectives in comparison with synthetic lubricants. The efficiency of bio-lubricants can be improved with appropriate addition of nanoparticles. Based on this prospect, the present study focusses on the assessment of viscosity variation in Neem oil with the mixture of ZnO nanoparticles. The ZnO nanoparticles are dispersed into the Neem oil at 0.25% weight concentration. Magnetic Stirring process is carried out in this study for the appropriate dispersal of ZnO in Neem oil. The variation in lubricant viscosity for 0.25% weight concentration of ZnO blended Neem oil is experimentally analysed at higher temperatures and sub-zero temperatures using a Modular Compact Rheometer. Herschel Bulkley model was set in the Modular Compact Rheometer for the viscosity analysis of ZnO blended Neem oil. A notable variation in viscosity is observed in the nano-biofluid developed as engine lubricant by the blended mixture of ZnO nanoparticles in Neem oil.