Today's automobiles have very specific motor oil viscosity requirements set by the manufacturer. Many cars use 5W-30 motor oil, but what do those numbers really mean? The first number in our example is 5W, which specifies that the oil must behave like a 5 weight oil (very thin and light) in cold conditions. It should flow freely below freezing so it can quickly reach unprotected engine components even after sitting for extended periods in winter conditions
The second number, 30 in our example, specifies that the oil must lubricate and resist breakdown like a 30 weight motor oil, which is quite a bit heavier and more viscous than a 5 weight motor oil. This indicates that it should be able to withstand the punishment of summertime driving without failing to properly lubricate the engine.
And what about the "W"? The "W" signifies winter, and is leftover from the days when cars had to run lighter oil for winter driving. Today it's just used to indicate the low temperature performance characteristics of the motor oil.