Synthetic fluid does last longer than standard motor oils, but I've found that a lot of car guys change their oil every 3,000 mi when you using non-synthetic. The manual says to have your oil changed every 5,000 mi and that is for standard oil; so I'd say you could go to about 7,000 mi on a high quality synthetic.
But if you are more anal or just like taking that much more care for your auto, like a lot of people on the site, I would change standard oil at 3,000 mi and synthetic at 5,000 mi. In the end, synthetic oil will last longer than standard and allow for longer periods between oil changes.
No matter what brand of oil you use, don't do anything to void the warranty. Sure, synthetics last longer, but if the owners manual calls for an oil change every 5000 miles, then do it anyway even though synthetics can last longer.