Oil is rated by the Society of Automotive Engineers (SAE) for viscosity. Viscosity is a rating which describes the thickness of a fluid. Engines are designed to use oil rated at a certain viscosity. Viscosity is very important in preserving engine life and reducing wear-and-tear.
To determine viscosity a measured amount of oil is heated to 100 degrees Celsius (210 degrees Fahrenheit) and poured through a device called a viscometer. The time it takes the oil to flow through the viscometer determines the weight of the oil.
Oil is often referred to in terms of weight. Oil weight is not to a measurement of ounces or pounds but viscosity. The terms weight and viscosity are interchangeable.
High-viscosity oil will have a higher number and consequently take longer to flow out of a viscometer. Fifty-weight oil is thicker and will flow out of the viscometer more slowly than 20-weight oil.
Modern oils sometimes have a multi-weight rating. Oil rated at 5W30 would be an example of multi-weight oil. Multi-viscosity oil is tested at colder temperatures as well as higher temperatures so it performs like 5-weight oil at colder temperatures and 30-weight oil at higher temperatures.
When an engine is cold, 30-weight oil will flow slowly and not circulate quickly enough to limit damage to the engine. Having multi-viscosity oil in your engine will allow the oil to be thin enough to circulate readily in a cold engine yet retain its needed thickness when the engine warms.
