Oil analysis is your friend! Get a Wix 24077 kit or 3; they're about $22 each on Amazon, but my local Auto Value has them at $15. Next time you're due, pull a sample and send it in. You'll get valuable information on your oil's condition (fuel/coolant/water contamination, additives, etc.) and a recommendation on whether to change the oil or resample at the next service interval. In my Jeep Liberty CRD, I was changing the filter at the recommended 6,750 miles and the oil every 13,000, and could have gone further. With a kit and the postage being about the same price as 2 quarts of oil, it's worth having the detailed info. Also, the reports come with the data from previous tests, and graphs showing change over time for the critical data points.