What does 15w_40 mean for engine oil?
4 Answers
W stands for WINTER, and the number before W represents the freezing point temperature. The freezing point temperature corresponding to 15W engine oil is -20℃. The number after W represents the kinematic viscosity of the oil at 100℃, with a higher number indicating higher viscosity. 40 indicates that at 100℃, the kinematic viscosity standard is between 12.5mm2/s and 16.3mm2/s. Below is a related introduction to engine oil: 1. Introduction to engine oil: Engine oil is the lubricating oil used in engines, composed of base oil and additives. The grade after the oil classification indicates its viscosity specification. The smaller the number before W, the better the low-temperature fluidity of the oil, indicating a lower usable environmental temperature and better protection for the engine during cold starts. The number after W is an indicator of the oil's high-temperature resistance, with a larger number indicating better protection performance at high temperatures. 2. Viscosity: Viscosity reflects the internal friction of the oil and is an indicator of the oil's oiliness and fluidity. When selecting lubricating oil, an appropriate viscosity and high-temperature viscosity index should be chosen to ensure that the oil has sufficient high-temperature viscosity to guarantee lubrication and sealing during engine operation, while also having sufficiently low viscosity at low temperatures to ensure cold-start performance.
Having driven for over a decade, I've found that engine oil viscosity grades are like clothing sizes – the numbers 15w40 represent cold-start performance and high-temperature protection. The '15' in 15w indicates the oil maintains fluidity at -20°C, ensuring easier cold starts; the 'w' stands for 'winter'. The following '40' denotes the oil's viscosity at 100°C – higher numbers mean thicker oil that forms a robust protective film during high-speed engine operation. However, most modern vehicles now use lower-viscosity oils like 5w30 or 0w20, while 15w40's higher viscosity suits older engines or heavy-duty vehicles frequently making long hauls. Always consult your maintenance manual when changing oil, as using the wrong grade may increase fuel consumption or even damage the engine.
As a mechanic with years of experience in an auto repair shop, I often explain engine oil grades to customers. 15w40 is a multi-grade oil, where the numbers before and after the 'w' correspond to cold-start and high-temperature viscosity respectively. The 15w indicates low-temperature performance, meaning it can still pump smoothly at -20°C; the 40 is the high-temperature viscosity grade, with higher numbers indicating stronger oil film at high temperatures. This type of oil is suitable for high-mileage older vehicles or work trucks that frequently carry heavy loads, as its thicker consistency helps fill wear gaps. However, it's important to note that newer engines generally use thinner oils to improve fuel economy, and blindly using high-viscosity oil can actually increase resistance. When choosing oil, always consider the SAE standard and manufacturer certifications, not just the viscosity numbers.
I remember my driving instructor said that engine oil grades are like dual-identity codes. The '15w' in 15w40 represents winter fluidity, where 'w' stands for winter, and '15' corresponds to a minimum applicable temperature of -20°C. The '40' indicates high-temperature protection capability – the higher the number, the thicker the oil becomes when the engine heats up. This type of grade belongs to multi-grade oil, suitable for both winter and summer without frequent oil changes. However, I found through research that most new cars now recommend 5w30, because 15w40 has slightly higher resistance during cold starts, potentially increasing fuel consumption by about 2%. But if the car is over 10 years old or frequently driven on mountain roads, using higher viscosity oil can actually reduce engine noise.
