It really depends. My VW diesel uses a special 5W40 (Castrol's VW 505.01 oil) oil that will go 10,000 miles between changes. But it's about $75 to do the change at the dealer.

If it's all highway, you can extend the change interval some. Highway driving is easier on engines than short stop-and-go trips. And extending the interval will be less wasteful, but you'll pay more for it (of course).

I'd probably use the recommended interval specified in the manual for "normal" (not "severe") driving, and use the highest API rated conventional oil I could find. But it may be that a synthetic that lets you extend the interval would be cheaper over the long run. (Note, though, that you apparently often have to add "make-up" oil when using [link|http://neptune.spacebears.com/cars/stories/interval.html|synthetics].)

HTH a bit.

Cheers,
Scott.