For those of you that have a problem with understanding various engine oil weight (or viscosity), Keep in mind that maufacturers are concerned with overall fuel mileage for their model line-up.
Most manufacturers are recommending 5w30 (some new Ford's and Honda's list 5w20) due to better fuel mileage. Lighter does not necessarily mean better wear protection. If you have cold weather concerns 5w30 may offer a marginal advantage at start up.
Oil manufacturers generally start with the lighter viscosity base oil and blend viscosity index (VI)improvers to allow the oil to retain it's "thickness" as the oil heats up. The lighter the starting point the more VI improver needed to get the higher reading. You generally don't see 10w40 anymore because people found out that the VI improver did not retain it's effectiveness over time (generally referred to as "shear stability") Some engineers have the same concerns over 5w30... hence 5w20 products are starting to appear in the marketplace.
Synthetics are inherently more stable due to their molecular structure and may offer some improvement under extreme conditions, (High heat, extreme cold, towing, etc.)
If you are still awake ... I like a 10w30 full synthetic. Be advised there are significant differences between synthetics due to the fact that the American Petroleum Institute (API) will not define "synthetic". Castrol has used this "loophole" to claim that their reformulated Syntec is a full synthetic. Mobil 1 is formulated with PAO's while Castrol uses a "highly refined" Group III paraffinic base oil to formulate their product. Personnaly I prefer the Mobil type "synthetic" product. I am sure this post may be boring to many readers but be advised that engine oil technology is moving at a fast pace due to manufacturers needs for improved fuel economy. You will soon see 0w10 or 0w20 products to meet their demands.