Viscosity index
Viscosity index is a measure of how well a liquid, like oil, can resist getting thicker or thinner when it gets hotter or colder. Think of it like the molasses or honey in your kitchen. When you try to pour it out of the bottle, it can be really sticky and slow, right? That's because it has high viscosity. But when you heat it up in the microwave, it can become more fluid and easier to pour because its viscosity has decreased.
Oil is similar in that when it gets hot, it can become thinner, which isn't good for your car engine. On the other hand, when it gets cold, it can thicken and become harder to move, which can be a problem for your engine too. So, the viscosity index tells us how much the oil's thickness changes as temperature changes. The higher the viscosity index, the less the oil's thickness will change with temperature - this can help keep your engine running smoothly over a wider range of temperatures.
Oil is similar in that when it gets hot, it can become thinner, which isn't good for your car engine. On the other hand, when it gets cold, it can thicken and become harder to move, which can be a problem for your engine too. So, the viscosity index tells us how much the oil's thickness changes as temperature changes. The higher the viscosity index, the less the oil's thickness will change with temperature - this can help keep your engine running smoothly over a wider range of temperatures.