| Every automobile has specifications for the types of motor oil that can be used in its engine. Most people just look for the bottle printed with the numbers that match those recommended in the owner's manual and leave it at that, without understanding what they mean. Doing so probably won't damage your car, but understanding those numbers certainly can't hurt.
The most common way of classifying motor oils is by viscosity. A lubricant that's too thick, or worse, not thick enough, can cause major damage to the moving parts of an engine. The SAE (Society of Automotive Engineers) sets the viscosity standards for these oils. On a container of motor oil you'll typically see SAE followed by a number, probably a hyphenated string containing the letter "W."
Here's what that number means. An SAE followed by a single number, for example SAE30, designates a single-grade oil designed for ordinary (non-freezing) operating temperatures. A higher number indicates a higher viscosity, but I couldn't find any reference as to whether the relationship is linear. A single-grade oil containing a W, for example SAE10W, would be made specifically for use in cold climates - the W stands for winter. Multigrade oils are dual-purpose; for example, SAE10W-30 functions as 10W in cold temperatures and as 30 at high temperatures.
The SAE specifies eleven grades: SAE0W, SAE5W, SAE10W, SAE15W, SAE20W, SAE25W, SAE20, SAE30, SAE40, SAE50 and SAE60. Multigrade oils functioning as a combination of W and non-W grades comprise almost all of the motor oils sold for use in passenger automobiles. | 
