Fact is, it doesn't matter how often you change your oil. You can do no harm, only good by changing your oil often. I lived in the USA for a while and a lot of those guys almost have an oil change each time they fill up with fuel. I change mine every 5000 miles with quality synthetic oil. If you do this, you can get half a million miles out of a straight six.
Always change via the sump though and it is better to do it after a journey of about 20 miles minimum to heat the oil, then have the car stand for 20 minutes to let it all drain to the sump, but it is still warm enough to drain every drop out.