What Do Those Numbers on Motor Oil Cans Mean?

Posted Thursday, Nov 22, 2018

So, you’re at the auto parts store to buy motor oil for your car, and you’re wondering what oil you should choose. But, what in the world? What do those numbers on motor oil cans mean? 10W-30, SAE 30, or 5W-30? Luckily, the employees there can look in their databases to see what oil weight, which is what the numbers represent, your particular vehicle requires. Also, it will be listed in your vehicle owner’s manual, but the easiest is to look on your oil fill cap- most cars have the required oil weight printed there. However, you may still want to know what the numbers mean, so let’s look at oil weight, viscosity, and those pesky numbers.

First of all, let me answer this question in two ways: the short, simple answer and the long, technical answer. If you want the short answer, you can stop reading after this paragraph. If you want the nitty gritty (not the best term for oil) on oil weights, read the entire article. Those numbers on motor oil cans refer to the oil's weight, which really refers to how thin or thick the oil is. A lower number is thin like cooking oil while a higher number is thick like molasses. One thing to keep in mind is that oils change thickness depending on temperature. You’ve all heard the adage of molasses in winter… Some motor oils only have one number (SAE 30, for example), so those oils are pretty thick when the engine is cold and pretty thin when it’s hot. Other oils have two numbers; they’re relatively thin even when cold and still somewhat thick when hot. In other words, they are less affected by heat than an oil with only one number. Hence, they perform a better job of lubricating your engine across all temperature ranges.

Now for the technicalities about those numbers on motor oil cans. The proper term for the thickness of oil is viscosity. Viscosity actually refers to the resistance of oil molecules to slide around each other. If you could see oil molecules, you would note that they are long carbon chains; I suppose you could view them as spaghetti noodles. (Interestingly, water molecules resemble balls or beads, so that’s why oil is “thicker” than water.) Thus, an oil with a higher number on the label has a higher viscosity, meaning its molecules slip less easily around each other. But how do those specific ratings on oil containers reflect viscosity? Long ago, the Society of Automotive Engineers (SAE) decided to rate motor oils. The method used was one in which a certain amount of oil has to flow through an orifice of a certain diameter during a certain amount of time. Depending on the size of the orifice or the time taken to flow, the oil would receive a certain number representing its weight. Through this procedure, the SAE was able to classify motor oils. But remember, oil viscosity changes with temperature, so the SAE developed two scales: one for cold (tested at 0 Celsius) and one for hot (tested at 100 Celsius). Hence, they rated some oils for summer use, like SAE 30, and some for winter use, like 20W. (The W is often erroneously thought to mean weight, but, in reality, it means winter.) Therefore, in decades gone by, you would run one oil in the warm months and another in the cold months. Otherwise, your oil would be fine in the winter, but then be so thin in the summer that it would not lubricate your engine. Conversely, it may be fine in the summer, but then be so thick in the winter that it could not be pumped through the channels of your engine block. Well, having to change oil for the seasons was somewhat annoying, so oil companies came upon an interesting idea: develop an oil that behaves like winter and summer oil.

How can an oil not follow the normal laws of physics as we’ve come to expect, though? Long ago, Newton mathematically described the relationship between a fluid’s viscosity and its temperature/ pressure, a relationship that almost all fluids follow. However, some fluids, referred to as non-Newtonian fluids, change viscosity in drastic ways at certain temperatures/pressures.
(You can experiment with non-Newtonian fluids yourself by mixing water with enough cornstarch to make a syrupy liquid. You can slowly push your finger into it like you would syrup, but if you try to jam your finger into it, it resists like concrete- be careful!) By adding polymers to oil, the developers created an oil that doesn’t follow the normal viscosity/temperature curve; they developed non-Newtonian fluids. For example, 5W-30 acts like 5W when the engine’s cold- it’s thinner than SAE 20 would be at startup, but acts like SAE 30 when the engine’s hot- it’s thicker than SAE 20 would be at operating temperature. Now you have the convenience of using one oil year round. And now you understand those numbers on motor oil cans.

Photo courtesy of reasonablerides.com