Did some research for the same question on another forum...
Motor Oil Viscosity
I'll quote, and stand corrected... However, go view the article.
Multi-viscosity grade oils have a wide viscosity range which is indicated by a two-number rating. Popular multi-viscosity grades today include 0W-20, 0W-40, 5W-20, 5W-30, 10W-30, 10W-40 and 20W-50. The first number with the "W" refers to the oil's cold temperature viscosity rating, while the second number refers to the oil's high temperature viscosity rating.
Note: Motor oils that have a wider range viscosity rating such a 5W-30, 5W-40 and 0W-40 are blended with more base stocks and additives.
Because of this, it may be harder for a wider range oil to remain in grade as the miles accumulate (which is why GM does NOT recommend using 10W-40 motor oil. They say it breaks down too quickly and does not say in grade as long as 10W-30 or 5W-30. Also, an oil with a lower winter rating like 0W-20 or 5W-20 will contain a higher percentage of thinner base stock oil (which is typically a synthetic oil). This requires more viscosity improver additive to achieve a the same high temperature rating as a 10W-30, 10W-40 or straight 30 or 40 weight oil.
Most vehicle manufacturers today specify 5W-20 or 5W-30 for newer vehicles for year-round driving. Some European car makes also specify 0W-20, 0W-30, 0W-40 or 5W-40 for their vehicles.
Always refer to the vehicle owners manual for specific oil viscosity recommendations, or markings on the oil filler cap or dipstick.
Always
use the motor oil viscosity recommended by your vehicle manufacturer. Using a different viscosity (thinner or thicker) may cause oil pressure and oil supply problems, especially in late model engines with cylinder deactivation and/or
variable valve timing (VVT).
As a rule, overhead cam (OHC) engines
typically require thinner oils such as 5W-30 or 5W-20 to speed lubrication of the overhead cam(s) and valve-train when the engine is first started. Pushrod engines, by comparison, typically specify 5W-30, 10W-30 or 10W-40.