Rheological, tribological and fuel economy assessment of low-viscosity engine oil varying base stocks and friction modifier

Purpose This study focused on the formulation and evaluation of low-viscosity grade API SN/GF5 0W-20 Passenger Car Motor Oils (PCMO). Design/methodology/approach Three candidate formulations were prepared for comparative study evaluation using different base oil compositions: fully synthetic oil, mineral oil and a blend of mineral and synthetic oils (semi-synthetic oil). Each formulation was enhanced with similar additives with friction modifiers (FMs). Molybdenum dithio carbamate is common in all three formulations; additionally, an organic FM was added to the semi-synthetic oil to assess their individual and synergistic effects on tribological behaviour and fuel economy performance. Physicochemical properties such as low-temperature fluidity, high-temperature high-shear viscosity, Noack evaporation loss, oxidation resistance and deposit-forming tendency (thermo-oxidation engine oil simulation test-moderately high temperature) were evaluated. Tribological characteristics were assessed using SRV, wear scar diameter and mini traction machine methods for all three candidate oils. Fuel economy performance was tested using the modified Indian driving cycle, comparing the third lab-blended semi-synthetic candidate oil (API SN/GF5 0W-20) with a commercial 5W-30 reference oil. Findings The results demonstrated that the lower-viscosity 0W-20 oils provided superior fuel economy compared to the 5W-30 reference oil. Among the formulations, the one containing the semi-synthetic fluid exhibited better efficiency performance. These findings underscored the importance of base oil selection and the synergistic effects of FMs in optimizing PCMO formulations for cost-effective and improved energy efficiency. Originality/value This work provides valuable guidance for lubricant formulators, OEMs and researchers by linking laboratory performance indicators to actual fuel-economy gains, advancing the development of next-generation energy-efficient engine oils.