The effect of adding MWCNT-ZrO2 nanoparticles (30%-70%) to engine oil (5w50) was studied to improve the performance of engine oil at low and high temperatures. The laboratory examination of this nanofluid was carried out at solid volume fractions of 0.05% to 1% and temperature range of 5 degrees C to 55 degrees C. Studies have shown the superiority of this nano-engine oil compared with produced nano-engine oils reported in literature and those are available in the market. The highlight point of this study is a 20% decrease in nano-oil viscosity (at solid volume fraction of 0.05%) compared with base oil. Also, the viscosity of the nano-oil was reduced in comparison to pure oil, in solid volume fractions below 0.75%, which is very useful and valuable in cold seasons and at low temperatures. (C) 2019 Elsevier B.V. All rights reserved.