What Do the Numbers on Oil Mean?

Have you ever stared blankly at a wall of motor oil in the store, wondering what all those numbers and letters mean? SAE 5W-30, 15W-40, 10W – what does it all mean? And does it really make a difference which goes in your car?  The numbers you see on your motor oil can range from 0 to 60 and are determined in laboratory tests by the Society of Automotive Engineers (SAE).

These numbers refer to viscosity, or thickness, with the lowest numbers representing the thinnest, and therefore the most viscous oil. Also, the “W” stands for “Winter,” and refers to oil that maintains viscosity even in cold winter temperatures. The need for different grades of oil is readily apparent when the weather switches because the temperature has a dramatic effect on the viscosity of the oil.

For example, 40-weight oil may be perfect for a long road trip in the summer when the engine is nice and hot but turn to sludge in the winter. On the other hand, 5-weight oil may work best in the middle of winter when temperatures are in the single digits, but provide very little lubrication after the snow melts.

Because of this, most motor oils on the market today are multi-viscosity oils, meaning they are formed from different grades of oil and share the characteristics of both thick and thin oil. For example, a 5W-30 oil will have the viscosity of a 5-weight oil when temperatures are cold but work as well as a 30-weight oil when the engine heats up. If you’re not sure which grade of motor oil to use, consult your owner’s manual. Most manufacturers recommend something around 10W-30, but there are circumstances that may call for different types of grades. For example:

• Older vehicles. As engines age, their parts often wear each other down. As a result, there can be more space between the parts of a 100,000-mile engine than in the engine of a brand new car. For older vehicles, consider a higher oil weight to help fill in this extra space.
• Very cold temperatures. If you often take short trips and live in a very cold climate, consider a lower number. Thin oil can work through the engine quickly on a cold day, protecting its parts from the moment it starts.
• Overhead engines. Because the oil needs to travel to the overhead cams and valve train, most mechanics recommend a lower weight for overhead engines.