The most common comparison used when trying to explain viscosity in motor oil is that of water and molasses. For the purpose of explaining viscosity in a very basic way in the automotive application we can define viscosity as the resistance of a fluid to flow. In the comparison of the water and honey, the water, of course has a lower viscosity, since it has very little resistance to flowing.
With the example of water fresh in your mind, think about what would change the water’s ability to flow. The ultimate answer is a number of things but since we’re just trying to understand the ratings on a bottle of motor oil, let’s stay with the obvious one--temperature. Yes, as the water was cooled toward freezing it would be less apt to flow. Please hold that thought.
Now let’s consider the molasses which would have the higher viscosity or resistance to flow. It would conversely react to heat and flow more easily as its temperature rose. If you keep in mind that the purpose of motor oil is to lubricate the moving parts of the engine and at the same time not retard its operation you can appreciate the fine line that the lubricant must be designed to walk.
Staying with the water/molasses comparison think about the varying conditions in which a car’s motor operates. In the winter the oil may need to be thinner like water so it would not slow down the moving parts, while when it’s hot a thicker molasses like oil would be less likely to thin out.
This brings us to those numbers on the bottle of oil. The oil may be labeled 5W30. This is an example of multi-viscosity oil designed to perform well in both hot and cold weather. It eliminates the need to change oil according to the season of the year. Let’s take the 30 first. It tells us that the oil is more like molasses than water when tested at 100 degrees Celsius. At the other end of the spectrum is the 5W, it has more of the characteristics of a thin oil when tested at a much colder environment. When you see the 5 with the “W” think winter and the need for the oil to be thin when cold.
When an engine is first started there is a time before the oil starts flowing that is called dry running time. This is extremely stressful for the car’s engine; the use of multi-grade oil cuts down on this harmful dry running time..
There are many more involved explanations of motor oil designations but this can serve as a beginning. Of course, the first step is to follow your car manufacturer’s motor oil recommendations.