The 3K miles/3 month thing is mostly BS and marketing. Unless you live in a very harsh climate, or are VERY hard on your car, there is no reason to change the oil that frequently.
I had my oil analyzed after 5500 miles (Castrol GTX), and it was still going strong.
If it makes you nervous, change every 3K miles, but I'd say go longer. Then have your oil tested,
http://www.blackstone-labs.com and see how it's doing.
You'll save some money.
