You see those cryptic combinations of letters and numbers on oil bottles everywhere. What do they mean? 

To decipher them, you have to understand viscosity. Viscosity measures how much the oil can resist flow. In other words, if you tip an oil bottle over, how fast it spills out indicates its viscosity. The more viscous, the slower it moves.

To indicate viscosity, the SAE (Society of Automotive Engineers) developed a standard scale.

0W is thinnest ("W" stands for winter)
5W
10W
15W
20W
25W
20
30
40
50
60 is thickest

The recommended multi-viscosity oil you use in your vehicle is usually good all year round because it can respond to a range of temperatures. That's why you see two numbers on most oils.

For example: 10W30

This means the viscosity is at 10W when the engine is cold and 30 when the engine is hot.

Low viscosities are good for cold temperatures (hence the "W" association) because the oil is thinner. Thinner oil flows more easily and moves quickly. When you start a cold engine up again, oil needs to travel to the top of the engine, where it trickles back down. Since oil is vital to lubricating your engine, it needs to move quickly and low viscosity helps it do just that.

When your engine heats up, higher viscosities mean the oil won't thin out too much and will keep parts separated by a film of oil. So your oil is always maintaining a delicate balance: it needs to flow well when the engine is cold but also retain enough body at higher temperatures to keep metal parts lubricated and separated. To find out what oil grade is right for your vehicle, check your vehicle's owner's manual.