The development of a sustainable powertrain requires improved thermal efficiency. Reducing frictional power losses through the use of ultra-low viscosity oil is one of the most effective and economical ways. To assess the potential for efficiency enhancement in a new generation of future engines using low-viscosity oils, a technical analysis was conducted based on numerical simulation and theoretical analysis. This study proposes a numerical method coupling the whole multi-dynamics model and lubrication model under mixed lubrication regimes. Then, load distribution was calculated numerically and verified experimentally. Finally, this paper compares the bearing load and frictional energy loss of the main bearings when using The Society of Automotive Engineers (SAE) 15W40 and SAE 0W20 oil. The results indicate that the application of ultralow-viscosity lubricant can reduce the hydraulic friction loss up to 24%, but the asperity friction loss would increase due to the reduction in load capacity. As a result, the design of a new generation of high efficiency internal combustion engines requires careful calculation and design to balance the trade-off relations between hydraulic friction and asperity friction.