I can't really talk about my truck, 'cause it doesn't see many km so it gets a change whenever I get around to it...
But for my daily drivers, whatever they are(Dodge Caravan right now), I typically run synthetic oil and change it every 10 000 km, which is like 6400 miles or so? I always grew up on the idea you change your oil every 5000 km or less, so I figure I can stretch it to twice that on synthetic.
edit - if you insist on running for 10 000 miles between changes, I would suggest changing your filter at 5000miles regardless of wether or not you find a bigger one that fits.