To add to above.
Basically ,without getting into lots of details...for a multi viscosity oil-- like 10w30 the 10 will refer to the abilility to flow(or thickness) at cold temps.. And the 30 is the lubricating properety (or thickness ) after it heats up...So a 5w30 will have better cold flow characteristics than a 10w30, but both will perform pretty much equal when heated up...It is important to stay with the recommended viscosity for the climate that the car is driven in because the engine bearings need both the ability for the oil to flow and lubricate them when cold and the protection that comes from the oil ability to maintain it's thickness when hot...
I use 5W30 all the time, but could use 10W30. It never gets cold enough to make any difference. In CT, it definately would. I got in the habit of using 5W30 when I lived in Albany, NY and ran it all the time, so I wouldn't have to remember what oil was in the pan.
I always ran 10w-30 in all of oil changes, then I steped up to 10w-40, and was just wondering since the winters coming, What would be best, so I guess 5w-30 then huh?, Ok Im going for it, thanks a bunch BRO!, Take Care!
I have not kept up with the new tech.on oil but used to be the lower # is the base viscosity of the oil polymers are added that coagulate under heat and produce the higher viscosity,originally designed for cold weather starts.The polymers continually break down so that in 2000 mi. you are back to base viscosity,so oil changes more often in winter.I was told that by a chevron engineer,he brought his ferrari in for us to work on.In the past I worked on exotics,mostly Italys best.
I moved on to warmer climates and only run straite grade oil and one viscosity higher after 50,000 mi.Thats in,Hawaii.Now in ca.and run 20-50 yr. round.I am starting to consider synthetic.
If it gets really cold where you are, I would suggest going with Castrol Syntec 0W30.
It is affectionately known as "German Castrol" because it is made in Germany. If you ever check out BITOG (Bob Is The Oil Guy - http://theoildrop.server101.com/forums/ubbthreads.php)
It has much information about oil, and all that is related. I would run either 0W30, or 0W40. And a high quality filter....you are all set!!!
Honestly you don't need 0w-30... the manual calls for 5w, id stick to that. I use 5w30 mobil 1 syth in winter, it gets to -30*C(-22*F) here quite often and I have no problems. Synthetic flows much better cold than dino oil, so that and 5w will provide you with all you need for winter.
Also It is good to go up to 10w30 in summer... as jakestune said they add polymers to get the various properties needed for each grade of oil. These wear out eventually so it is best to get a oil with "closer" numbers... eg: 10w30 needs less polymer additive than 5w30... 10w30 relies less on the additive so when it wears out it's not so bad.
I'd lways stay in the recommend range though, unless your in some extreme conditions