Our new Mazda CX9 recommends changing oil every 7,500 miles, or 5,000 miles for rough use. The dealer said that around here (WNY) all use is considered rough use because of the cold and salt; so they say to change every 5,000 miles.

The manual calls for 5w20. My wife went into for an oil change and her recollection was that that it needed synthetic 5w20, even though I told her yesterday that was not what was required; but of course she got synthetic.
So my question (at long last)... As long as we paid through the nose for synthetic, can we stretch out the next oil change to 7,500 miles? Isn't synthetic supposed to last longer?

And a second question... I was reading about this and an oil company(Amsoil,whoever they are)says that the recommendation for 5w20 is so that the car company can claim another 0.1mpg in their mileage reports, and that 5w-30 is actually better for the engine.  Sounds like nonsense to me, but the oil company also sells 5w-20, so it is hard to see why they would lie. Whatcha think?