I do Mobile 1 every 5K miles - I also top up every 1000 miles (uses a little less than a quart per 1000 so about a quart per 5K). If you do decide to do 5K changes, try to line up your changes to hit right on the 5's and 10's - it's easier to keep up with the changes that way (plus you'll know ever time the 1000 rolls over that it's time to check the oil and top up).

Also, I buy the 6 pack of Mobile 1 at Costco. It gives you your change plus the quart for top-ups and ends up costing about the same as 5 quarts so you get the extra for free.

-- John