DO NOT waste you time with a synthetic blend. You are just paying more for something that can contain as low as 2% synthetic oil... yes that's right 2%. Since it is listed as a blend there is no required percentage of synthetic and conventional oil, these blends have been tested and have shown to be that low.

This website lists some differences http://www.synlube.com/oil.htm

Anyways synthetic oils have low volatility (does not change it's properties as easily as conventional), good inherent viscosity indexes, good lubricity, good low temperature flow, and high stability at high operating temps. You may not notice the difference yourself but your engine sure will.

http://www.bobistheoilguy.com/

That website has more information on oils and other additives than anyone else could tell you on here, proving the facts with oil analysis. It's alot of reading, but atleast you will not end up paying more for nothing.

So to answer your questions in short:

The life span does not matter, synthetic oil will hold up longer, but it is recommended that you change your oil at regular intervals if you want to keep everything running smooth.

Synthetic oils will perform even better in winter than conventionals, they do not gel up like conventionals. Also in areas that are around -25 and below on average during winter time "0 w30" would be an even better recommendation. All the police cars here in Winnipeg are changed to that during the winter, because it is proven that any 5 and up starts to fail and gel up horribly in those low of temperatures.

no gain at such a new car

I used Castrol super syn.It is better oil but cheaper and it performs well year round.

i was prefer the dealers to change it for me.

Synthetic oil maintains consistent viscosity even in very high heat and, since most engine wear occurs at start-up, works to offer more lubrication at this crucial stage. The bottom line is that synthetic oil use leads to longer life, more efficiency and enhanced performance for your engine.