Answer Geek: Those Motor Oil Numbers

-- 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.

But not all liquids behave in accordance with the dictates of Newtonian Viscosity. There is a class of fluids for which viscosity does not change in a linear fashion in reaction to pressure or heat. These rebels of the liquid world are known as non-Newtonian fluids. In such liquids, for example, viscosity may decrease as temperature rises until a critical point is reached and then the liquid may suddenly become more viscous.

What does this have to do with the numbers on a quart of oil, you ask? One of the tricks for making good motor oil is that its viscosity must be low enough so that it will flow when cool, but not so low that it fails to lubricate at high temperatures. Most pure petroleum lubricants are Newtonian fluids, and at either the top or bottom of the temperature range, they just don’t work well any more. In recent years, engineers have discovered that adding certain carbon polymers to petroleum lubricants will turn them into non-Newtonian fluids that are much better at protecting a car engine under a wide range of conditions. Those polymers are called viscosity modifiers, and motor oil makers have learned to add the just the right combination of viscosity modifiers to create lubricants that flow easily at very low temperatures while maintaining enough viscosity to lubricate the moving parts in an engine at very high temperatures.

Viscous Cycle

So, those numbers on a quart of oil? They refer to oil viscosity, based on a scale established by the Society of Automotive Engineers (which is where the “SAE” comes from). The scale rates oil from a low of 5 to a high of 50. As you’ve probably noticed, most automobile motor oils have two numbers. These are multi-grade oils, which means they are non-Newtonian fluids.

The first number describes viscosity at low temperatures. The second number refers to viscosity at normal engine operating temperatures. Some examples: the “5” in a 5W-30 motor oil will protect an engine down to an air temperature of about -25 degrees Fahrenheit. The “10” in a 10W-30 is good if the lowest temperature where you live is likely to be in the neighborhood of -10 or so. As for the “30,” that is well-suited for the typical temperatures that most cars operate at these days. If you are driving a racing car, or pulling a heavy trailer on a hot summer day, you might want the higher-temperature protection afforded by a motor oil with a second number of, say, 50. (As for the “W,” it indicates that the oil is designed to work well in cold weather.)

In the old days, motor oils came in only the single grade variety. Back then, if you lived in a cold climate, you’d would have to use a different oil grades in different seasons, switching from an SAE 30 in the summer to an SAE 10W in the winter.

Which oil should you use? Don’t ask me. Like I said, I’m no gear head. My suggestion: Refer to the owner’s manual that came with your car. It will tell which grade to select. Personally, I wouldn’t get too creative about my choice of motor oil unless you know a lot more about cars than this Answer Geek does.

And my answer the next time someone asks me what kind of oil I need for my vehicle? First I’m going to ask them if their selection of motor oils includes any non-Newtonian fluids. Then I’m going to inquire about centistoke values. After that, I’ll give them a withering stare to make up for all the times that I felt like an idiot when I thought “W” stood for “weight” and I didn’t have clue what grade of oil my car required. Finally, I’ll ask for 10W-30 oil because that’s what my owner’s manual recommends. That is, if I don’t get nervous and forget.

Todd Campbell is a writer and Internet consultant living in Seattle. The Answer Geek appears weekly, usually on Thursdays.