How to Identify Engine Oil Viscosity Grades?
3 Answers
10W-40 is the SAE viscosity grade of engine oil. This viscosity value first indicates that the oil is a multi-grade oil. The symbol W stands for winter. The smaller the number before W, the lower the low-temperature viscosity, the better the low-temperature fluidity, and the lower the applicable minimum temperature. The number after W is an indicator of the oil's high-temperature resistance. The larger the number, the better the oil's protective performance at high temperatures.
The first time I learned to identify engine oil grades was when my old car developed louder engine noises after using the wrong oil. Now when I check the label on oil bottles, I mainly look for two parts: the SAE viscosity grade, like numbers such as 5W-30. The number before the W represents cold-start fluidity—the smaller the number, the thinner the oil, making it easier to start. The number after W indicates high-temperature viscosity—the larger the number, the thicker the oil, making it more suitable for high-temperature environments. The other part is the API service category, with letters like SN or SP at the beginning, indicating the oil's performance standard—the higher the letter, the better the oil quality. I recommend buying oil based on the model recommended in the vehicle manual rather than randomly choosing by feel, as improper selection can lead to increased wear or higher fuel consumption. I also remind new car owners to consider seasonal changes when changing oil: choose a lower W number in winter to prevent starting difficulties and a higher number after W in summer for better heat resistance. Regularly check the dipstick to ensure the oil is clear and not blackened.
As someone who frequently works on cars, I find oil grades quite straightforward: just look at the large print on the front of the bottle—the SAE number string indicates viscosity grade, such as 0W-20, which is thin oil suitable for new cars. Then check for the API mark, like SP, which stands for high performance and engine cleaning. Don't forget to check the fine print on the back of the bottle, as it might include ACEA or ILSAC standards, but SAE and API are the primary indicators. Before changing the oil, I always cross-reference with the manual to ensure compatibility and avoid engine damage. Choosing the wrong oil can accelerate carbon buildup or increase noise, so I recommend DIY beginners keep a record of each oil change model to build good habits. While professional oil testing at maintenance shops is reliable, learning to do it yourself saves money and hassle.
