The first number has to do with the viscosity, the ability to flow when at a cold temperature. The second number has to do with the amount of protection when hot, the thickness of the oil film.
There is a common belief that the lower the "w" number, the better the gas milage. I'm not sure if that's always the case. I'm personally a believer that a synthetic will give better MPG than a non of the same weight. Especially if you factor in the fact that all oil will thicken with use but synthetic will thicken more slowly.
Here are a few of my opinions that are of course open to correction:
-Synthetic longer lasting than non.
-Synthetic less friction than non.
-A 10w in the warmer south will flow as well or better on cold start as a 5w will in the cold north.
-A 5w will drain off the engine parts more after shut down than a 10w, leaving less oil protection at start up. (I'm less sure about this one so flame away.)
-The difference in using a 5w up north vs. a 10w down south is minnor and will not be any harm.
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