Viscosity definition
A measure of an oil's ability to pour or be thick. Every container of oil is marked with the viscosity of the oil. Straight weight (also called "single viscosity" or "single weight") oil has a single number like 30 weight. A lower number means the oil is thinner and is particularly good for lower temperatures. A higher number is better for hotter temperatures. Multigrade (also called "multiviscosity" or "multiweight") oil has two numbers like 10W40. When the temperature is low, a multigrade oil like 10W40 will act like a 10 weight oil; but when the temperature increases, it will act like a 40 weight. In this way, a multigrade oil is excellent for all-around driving.