This is what my friend says:

A 5 wt. takes less energy to "sling around" and therefore you will get a small amount of energy savings. My GMC manual recommends a 5w30 engine oil. That is a "northern mentality". Look on the temperature range in this area of the U.S.A. and temperatures ranging from 0*F - 100*F+ can use a 10w30 engine oil. Oils get thin when they get hot. a 10wt. provides more cushion between the moving parts and couple that with our solid lubricant moly to help eliminate wear and reduce friction... it makes for a good match.
You won't find anything as "slippery" and you have the option to go with a base oil from 100% parraffinic, to semi-synthetic, to 100% pure PAO Synthetic.