Oil Viscosity

Viscosity is a measure of how easily the oil flows.

© iStockphoto.com/Georgi Roshkov

It's intimidating when you see those bottles of engine oil lined up along the shelves, each promising to keep your engine cleaner, improve your fuel efficiency and more. And every one of them has cryptic letters and numbers on the front.

Those letters and numbers tell you what the oil's viscosity rating is (some people call this the oil's weight). Viscosity is a measure of how easily the oil flows -- is it thick or thin? The Society for Automobile Engineers (SAE) tests all engine oil at 210 degrees Fahrenheit (98.9 degrees Celsius), a typical engine operating temperature, and gives it a rating from 20 to 60. On the bottle, it will say something like SAE 20 or SAE 30, two common viscosities.

If you live in a colder climate, the label will be a little different. It will likely say something like 5W-30. The "5W" means that the SAE has tested the viscosity of the oil at a colder temperature. This oil will be thinner when you start the engine on a cold morning but will perform like an SAE 30 oil at 210 degrees Fahrenheit (98.9 degrees Celsius), when you've been driving a few minutes and the engine is warm.

In any case, your owner's manual will likely have a suggested oil rating for your car. A trusted mechanic will be able to help you choose, too. Just remember that colder starts need a thinner oil at first, and warm engines need a thick enough oil to not disappear when the engine becomes warm.

Next up: The oil change.