What is the difference between 0W-20 and 0W-40?
2 Answers
The difference between 0W20 and 0W40 lies in their base viscosity and high-temperature shear resistance, as detailed below: Both 0W20 and 0W40 start with 0W, indicating they share the same low-temperature viscosity. The key distinction is the numbers following the W—20 and 40—which represent the oil's kinematic viscosity at 100°C. A 20-grade oil has a kinematic viscosity of 8-9 at 100°C, while a 40-grade oil ranges between 12.5-15. Higher numbers indicate thicker oil films at high temperatures and stronger resistance to shear. The high-temperature shear index for 20-grade oil typically does not exceed 2.9 mPa·s, whereas 40-grade oil maintains a shear index above 3.5 mPa·s. Additionally, 0W40 exhibits poorer high-temperature fluidity compared to 0W20. Oil viscosity changes with temperature, making it crucial to consider performance under both low and high temperatures. For example, in 5W-30, the first number denotes low-temperature fluidity—5W means it can withstand temperatures as low as -30°C. A smaller number indicates better low-temperature fluidity, ensuring smoother cold starts for the engine. The second number represents the oil's kinematic viscosity at 100°C, with higher values indicating better viscosity retention and lubrication performance at high temperatures. Better low-temperature fluidity facilitates smoother cold starts and enhanced engine protection. Different engine designs require either 20-grade or 40-grade oil. 20-grade oil is commonly used in newer Japanese, Korean, and American models, while 40-grade oil is preferred for German, high-displacement, and high-performance vehicles. For full synthetic oil, the recommended replacement interval is one year or 10,000 kilometers. Semi-synthetic oil requires more frequent changes—around 7,000 kilometers or every six months. When selecting engine oil, keep the following in mind: Choose oil based on the engine's requirements. Avoid using overly advanced oil for less demanding engines or lower-grade oil for high-performance engines. Opt for multi-grade oil whenever possible. Multi-grade oils are economical, long-lasting, and efficient, offering superior engine protection. Due to their properties, they may darken prematurely or exhibit lower oil pressure compared to conventional oils—both are normal. Since oil significantly impacts engine performance and lifespan, always adhere to the vehicle's manual for the specified series, service grade, and viscosity grade. The manufacturer-recommended oil considers engine performance and regional climate conditions, providing a reliable guideline with a substantial safety margin.
As someone who frequently drives in urban commutes, comparing 0w-20 and 0w-40, the former feels smoother during summer starts, produces less noise after ignition, and helps me save quite a bit on fuel costs, especially with noticeable fuel consumption drops in stop-and-go traffic jams. However, the 40 performs better during high-speed driving, offering better engine protection against overheating in hot weather or long-distance trips, reducing wear risks. I feel the low viscosity of 20 suits modern new cars—eco-friendly and economical—while 40 is better for older vehicles or machines often used for long distances. Don’t mess around when changing oil, or it could affect engine lifespan and driving safety. I’ve also found that regular oil quality checks and following the manual’s recommendations are smart choices.
