When using 5W-40 engine oil as a substitute for 5W-30, attention should be paid to the impact of their high-temperature viscosity difference on the engine. The kinematic viscosity of 5W-30 at 100°C is 9.3-12.5 mm²/s; it forms a thinner oil film with better fluidity, making it suitable for new vehicles or precision engines. It can reduce fuel consumption by approximately 3%-5% and enhance cold-start performance. In contrast, 5W-40 has a viscosity of 12.5-16.3 mm²/s, forming a thicker oil film with superior high-temperature stability. This makes it suitable for high-load driving conditions, turbocharged models, or older vehicles with over 80,000 kilometres mileage, as it helps reduce engine noise and compensates for wear gaps. Using 5W-40 in non-high-temperature environments or new vehicles may increase fuel consumption by 0.3-0.5 L/100km and result in higher cold-start resistance. German vehicles or those frequently driven at high speeds may consider 5W-40, while new Japanese/Korean vehicles or urban commuters are better suited for 5W-30. Incorrect selection may lead to issues such as inadequate lubrication or abnormal oil pressure.
