Don't let the quickie-lube guy or the dealer fool you. Unless you are doing "hard duty" driving (lots of stop and go, or high temperature, for example), you do not need to change your oil any more frequently than 5,000 miles. If you drive a higher mileage car (like myself), and your car loses some oil, you should keep an eye on it and add some as necessary.
I learned this some years ago from Consumer Reports, which did an oil study on a fleet of NYC taxi cabs, probably one of the harshest use profiles there is. CR found no additional engine wear between 3,000 mile and 6,000 mile schedule taxis. They recommend oil changes on 7,500 mile intervals, for the average driver, unless the manufacturer recommends otherwise.
The bottom line. Modern motor oils needn't be changed as often as oils did years ago. More frequent oil changes won't hurt your car, but you could be spending money unnecessarily and adding to the nation's energy and oil-disposal problems.
Even in the severe driving conditions that a New York City taxi endures, we noted no benefit from changing the oil every 3,000 miles rather than every 6,000. If your driving falls into the "normal" service category, changing the oil every 7,500 miles (or at the automaker's suggested intervals) should certainly provide adequate protection. (We recommend changing the oil filter with each oil change.)
Tom and Ray Magliozzi advise an oil change interval of 5,000 miles.
Consider the savings. Over 5 years, a 3,000 mile oil change schedule will cost you about $500 on oil changes ($25/per x 4/year). A 5,000 mile schedule will cost you about $300, and a 7,500 mile interval will cost you $200.
I learned about another controversial practice from an old-timer. I'm not brave enough to try it, but he swore by it: he changed his oil filter every 3,000 miles religiously, but didn't ever drain out the oil. Just topped it off at every filter change. He swore that he never had an engine go bad with this method. He claimed that the gunk would leave with the filter, and the clean top-off would keep the additives fresh, but I suspect this is pushing it. My understanding is that oils (and additives) do break down, and mixing in 1 quart of fresh oil with 4 or 5 quarts of tired oil is not going to be as effective as all new oil.
Don't send any extra money to the Saudis. Go by what your owner's manual tells you, not what the quickie-lube and dealer say. They want to sell you the service.