We’ve all seen 5W-30 or 10W-40 on oil bottles before, but what do they mean? Let’s look at how motor oil grades are classified and explain what the numbers on the bottle mean.
Viscosity is a fluid’s resistance to flow at a specific temperature. The Society of Automotive Engineers (SAE) assigns a viscosity number to oil based on its flow at 210 degrees F, which is roughly the standard operating temperature for most motors. The higher the number, the thicker or slower flowing it is, which changes the nature of how it coats internal engine components and protects against heat and friction. This means a 30 weight oil flows more quickly than 50 weight oil but doesn’t offer quite the same level of protection at higher operating temperatures or in stressful conditions.
The viscosity of motor oil is notated with the common “XW-XX.” The number preceding the “W” rates the oil’s flow at 0 degrees Fahrenheit (-17.8 degrees Celsius). The “W” stands for winter, not weight as many people think. The lower the number in front of the “W”, the less it thickens in the cold. So 5W-30 viscosity engine oil thickens less in the cold than a 10W-30, but more than a 0W-30. An engine in a colder climate, where motor oil tends to thicken because of lower temperatures, would benefit from 0W or 5W viscosity. A car in a hot climate would need a higher number to keep the oil from thinning out too much.
The second number after the “W” indicates the oil’s viscosity measured at 212 degrees Fahrenheit (100 degrees Celsius). This number represents the oil’s resistance to thinning at high temperatures. For example, 10W-30 oil will thin out at higher temperatures faster than 10W-40 will.
The grade of oil you choose should be in accordance with the manufacturer’s recommendations which can be found in the owner’s manual. The climate temperature will help you determine which motor oil grade is best for you. Come into Earp’s for your next oil purchase.