To the last person that posted, synthetic oil IS formulated for your engine. Car manufacturers in general don't put synthetic in at the factory becauses it is more EXPENSIVE. Some of the more expensive cars do come with synthetic oil right from the factory. BMW, MB, Porsche, Dodge Viper, Chevy Corvette, these are all I can think of right now. So it is primarily a $money$ issue with the auto manufacturers. It's cheaper to go dino oil for the manufacturer, its cheaper for the average consumer to use and it works ok for 90% of the driving and drivers out there.

That said, I would not recommend synthetic on an older engine if you have been using regular oil all along. It may cause your sseals to leak and it may cuse some problems becausse of deposits that have already built up inside your engine.

Advantages of synthetic:
Flows more easily, alowing for easier cold starts, less engine wear at start up and during warm up, better protection during very cold weather operation.
Has a higher temerature range without breaking down, alows better engine protection in very hot weather, especially stop and go traffic during the summer time.
Because it flows better, you will get a slight improvement in fuel economy and acceleration.
It keeps your engine cleaner because it does not promote deposit formation due to oil breakdown.
More stable viscosity range because the oil does not breakdown and become a single viscosity oil as dino oil does over time.
Allows you to take advantage of car manufactures extended oil change intervals because synthetic doesn't break down in "severe" use categories.

Disadvantages to syntetic:
More expensive
Not recommended for use in higher milage vehicles that have always used dino oil in the past
You still must change the oil in accordance with the manufacturers recommendations (6k - 7.5K)