Answer Geek: Those Motor Oil Numbers

<br> -- Q U E S T I O N: Motor oil: SAE 30, 10W-40, 20W-50. What do all those number and letters mean? — Danny S.

A N S W E R: Many thanks for your query. I may be an Answer Geek, but I’m no gear head, and it’s always nice to increase my limited store of knowledge about cars and engines. At the very least, Danny, answering your question will save you, me and others like us from that withering stare we get at the gas station or auto parts store when we ask for a quart of oil and then have to own up to the fact that we have no idea what grade of oil our car requires. For men, that is one of those dreadful moments designed to amplify whatever feelings of masculine inadequacy you may already be struggling with.

Here’s what I found out:

The thing to keep in mind about motor oil is that its main job is to protect your engine from wear caused by the friction created when moving parts rub against each other at high speeds and high temperatures. The quality that allows motor oil to lubricate those moving parts is viscosity, which describes the ability of a liquid to flow. In a laboratory, viscosity is usually assessed by measuring the flow of a liquid through a tube with an opening of a fixed size at a standard temperature, or by measuring the resistance that liquid exerts on a rotating shaft in a container. The standard unit of measure for viscosity is either the centistoke or the centipoise. Bigger numbers mean greater resistance to flow and higher viscosity. Water, for example, has a very low viscosity and it flows quite easily. Molasses, which is thick and goopy, has a very high viscosity. Needless to say, neither one is a good bet as a substitute for motor oil.

Before we get to meaning of the numbers on a quart of oil, you need to understand one more thing: Newtonian fluid dynamics. As you may or may not know, many liquids obey Newton’s Viscosity Law, which in simple terms states that viscosity varies directly in relation to external forces. Typically for Newtonian fluids, lower temperature or higher pressure raises viscosity in a straightforward way as friction increases between molecules. In these liquids, more friction means greater viscosity, plain and simple.