5W-40 engine oil has a higher high-temperature viscosity than 5W-30, which is the core difference between the two. Under 100°C operating conditions, the kinematic viscosity range of 5W-30 is 9.3-12.5 mm²/s. It has a thinner oil film and better fluidity, which can reduce fuel consumption by approximately 3-5%. In contrast, 5W-40 achieves a kinematic viscosity of 12.5-16.3 mm²/s, with oil film thickness and high-temperature stability improved by about 20%. It is particularly suitable for German turbocharged vehicles, older cars with over 80,000 kilometres mileage, as well as high-temperature environments or aggressive driving conditions. Performance-wise, 5W-30 provides quicker cold starts and superior noise control, while 5W-40 delivers enhanced anti-shear protection during high-speed operation or heavy loads but increases fuel consumption by 0.3-0.5 L/100km. 5W-30 is recommended for new Japanese and Korean vehicles, whereas 5W-40 is more appropriate for German cars or aged vehicles. Ultimately, the manufacturer's manual recommendations should prevail to prevent potential engine lifespan impacts from incorrect viscosity selection.
