There are reasons why you would do an extended drain interval especially on synthetic oils. First off Americans are brainwashed into thinking that 3,000 mile oil changes is the best thing for your motor. This is basically a marketing campaign generated from the service oil industry which includes oil changes places, car dealerships, and filter companies to convince folks to pad their wallets.
Today's oils, especially synthetics, are resistant to oxidation, thermal breakdown and oil sludging. They have superior protection against ash and other deposit caused by hot spots in the engine. With the introduction of Ultra Low Sulfur Diesel, acid byproducts have been greatly reduce, which allows for the engine oil to be in service longer - mileagewise and timewise. So even if you drive the car less than a 1,000 miles in a year, synthetic oil will still be serviceable and maybe even serviceable up to two years. If you look at an Oil Analysis report, you'll find the TBN (Total Base Number - a figure that measures the acidity of the oil) in a well acceptable range. You also want to look the insoluble number for soot.
Another reason to keep synthetic oil in there longer is to continually keep the additives bonded to the moving engine parts. The additives of the oil get bonded by high pressure to the moving components. This is basically where your best engine protection lies. These additives account for like 3% of the volume of engine oil, but produce 90% of the engines oil protection. With that low volume number, these additives only start bonding to the engine while being continuously circulated.
Here is an article on bob's the oil guys website about how one additive works, which is similar to the other types.
http://www.bobistheoilguy.com/moly-basics/
When you change oil often, you introduce an initial cleansing detergent. This detergent attacks and cleans out all contaminants. Though it does a great job of cleaning, the drawback is it also cleans off the existing additive bond and prevents the new additives from forming their bond to the moving parts until the detergent cycle is used up during the first 1000 or 2000 miles. The proof is in doing an oil analysis, several that have been posted in various forums show the engine oil's wear rate is actually higher at 3,000 miles, then it is at 7,500 mile and at 10,000 miles. By keeping the engine in there longer, the detergent cycle is reduce which leads to less wear on the engine.
Here are some SAE white papers that explains the benefits of extended drain intervals.
The Effect of Oil Drain Interval on Valvetrain Friction and Wear
Characterization of Anti-Wear Films Formed from Fresh and Aged Engine Oils
So in the long term you are saving money, saving resources, driving more and worry less.
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