In electric vehicles (EVs), drivetrain lubricants are often utilized not only as a lubricating oil for the drivetrains but also for motor cooling. As such, they are required to both improve the efficiency of the drivetrains and to have a high cooling performance. Both requirements can be met by lowering the viscosity of the fluid, which effectively improves the heat transfer coefficient, reduces churning loss, and improves efficiency. However, low viscosity may adversely affect the fatigue life of gears and bearings. To address these issues, we used a high-performance base oil and optimized additives (e.g., anti-wear agents) to develop a fluid with higher lubricity than conventional automatic transmission fluid (ATF), even though its viscosity is lower. The 100,000-kilometer (WLTC mode) endurance test on an actual vehicle confirmed that there was no damage in the reduction gear unit parts. We also evaluated the torque loss on the reduction gear unit and fuel consumption and confirmed a higher performance in each test. This fluid can be widely utilized in hybrid electric vehicles (HEVs), plug-in HEVs (PHEVs), and battery EVs (BEVs) worldwide, and its usage in fuel cell electric vehicles (FCEVs) will help contribute to improving fuel economy.