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In this study, the improvement in SAE 10W-40 engine oil tribological performance after the addition of magnesium oxide (MgO) nanoadditive and two different phosphonium-based ionic liquids (ILs) was investigated. Besides, the rheological behaviour of MgO-based nanolubricant and IL-based hybrid nanolubricants at the temperature range from 293.15 to 363.15 K was studied. The nanoparticle characterization was performed by means of transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The tribological properties, friction coefficients and wear parameters of the formulated oil modified with 0.01 wt% MgO and 1 wt% ILs compared with the neat 10W-40 oil were performed and analysed using a ball-on-three-pins tribometer and a 3D optical profilometer, respectively. Further analysis on the worn surface was shown by Raman spectroscopy and SEM images illustrating the formation of the protective IL and MgO tribo-films as hybrid additives. In friction tests with sliding steel-steel tribo-pairs, IL3-based hybrid nanolubricant decreased the coefficient of friction and wear volume by 7% and 59%, respectively, in comparison with the neat SAE 10W-40, hence better positive synergies were found for MgO and IL3 as hybrid additives. Interestingly, the thermophysical characterization by rheology also revealed that the nanoparticle and ionic liquids addition did not affect neither the viscosity response nor the Newtonian behaviour of the engine oil, adequately meeting the requirements for their use in internal combustion engines. Graphical Abstract

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