Careful with synthetic on high mileage engines....I've heard lots of stories where people have changed to synthetic on high mileage engines and all of a sudden things are leaking! If you have used dino oil for a long time stick with it. My car has 117k on it and I'm going to stick with the dino oil. Best time to switch to synethic is with low mileage on a engine with some breakin time -- maybe 10-15k?
Of course some engines are coming with synthetic from the factory now. Regardless, if your engine has a lot of miles on it I wouldn't reccomend the switch (IMHO).
'02 Aurora 3.5, factory NAV retrofit, CD changer retrofit, Philips Xenon lights. '96 SE: Gone but not forgotten :( Oh yeah, and a few other toys.