How Many Grades Are There for Lubricating Oil Viscosity?
3 Answers
According to the viscosity index, lubricating oils can be classified into three grades: 35—80 is medium viscosity index lubricating oil; 80—110 is high viscosity index lubricating oil; above 110 is premium viscosity index lubricating oil. Oils with a viscosity index between 100—170 are high-grade multi-grade lubricating oils, featuring smooth viscosity-temperature curve changes and excellent viscosity-temperature properties. At lower temperatures: The solubility of high-molecular organic compounds in these viscosity index improvers is low in the oil, causing the molecules to curl into tight small clusters, thus resulting in minimal increase in oil viscosity. At high temperatures: Their solubility in the oil increases, and the curled linear molecules expand and elongate, leading to a more significant viscosity increase, compensating for the viscosity drop of the base oil due to rising temperatures. Therefore, the higher the viscosity index, the smaller the viscosity change with temperature.
A vintage BMW came into the shop the other day, with the owner complaining about excessive engine noise. Upon inspection, I found the wrong lubricant viscosity had been selected. Simply put, viscosity grades are classified by SAE standards into two main categories: single-grade and multi-grade. Single-grade oils like SAE 30 or 40 are only suitable for specific seasons, while multi-grade oils, such as 5W-30 or 10W-40, are most common. The 'W' stands for winter, with a lower number before it indicating smoother cold starts, and a higher number after it indicating better high-temperature protection. In northern winters, oils starting with 0W are ideal, while in southern summers, viscosities above 40 are preferable. Older cars perform better with thicker oils like 15W-50, whereas newer cars benefit from thinner oils like 0W-20 for better fuel efficiency. Always check the manual before an oil change—using the wrong oil can range from wasting fuel to damaging the engine, so never experiment carelessly.
Last year, I drove my Passat on a long-distance trip, and the engine suddenly started humming on the highway. Later, I found out it was due to using the wrong viscosity grade for the engine oil. Viscosity grades are actually classified according to the SAE standard, mainly divided into single-grade and multi-grade forms. Single-grade oils have fixed ratings like 30, which are less commonly used now. Multi-grade oils, such as the common 5W-30, are more practical. The number before the 'W' represents low-temperature fluidity—5W protects the engine faster during cold starts compared to 10W. The number after the 'W,' like 30, indicates high-temperature viscosity, with higher numbers meaning thicker oil films. I remember one summer when I used 5W-20, and the oil temperature skyrocketed while climbing a hill, nearly causing engine seizure. Now, I always follow the manufacturer's recommendations. German cars generally use 40 viscosity, while Japanese cars are better suited for 20 or 30. When choosing engine oil, don’t just focus on the brand—wrong viscosity makes it all pointless.
