You should always analyze the source of your information because it can make a big difference if that source has your best interests in mind. If a used car salesman tells you something about buying a car, they may have their own agenda that won’t help you.
In the case of oil changes, I usually hear how often I should change the oil from the people I’m paying to change my oil. Changing your oil too frequently isn’t going to cause any problems (except to your budget) so it’s not bad advice, but is it really necessary?
I did some research and the short answer is it’s not necessary to change your oil as frequently as many shops recommend. Your best bet is to follow the manufacturer’s recommendations found in your owner’s manual.
There was an Ask Metafilter post on this, asking if the 3,000 mile oil change was a scam. The general answer there was yes, it is a scam. From a discussion at autoblog, several people recommend changing your oil every 5,000 miles.
Lastly, consumer reports car care myths vs. reality says,
Although oil companies and quick-lube shops like to promote this idea, it’s usually not necessary. Go by the recommended oil-change schedule in your vehicle’s owner’s manual. Most vehicles driven under normal conditions can go 7,500 miles or more between oil changes. Some models now come with a monitoring system that alerts the driver when the oil needs changing. Depending on driving conditions, these can extend change intervals to 10,000 or 15,000 miles.