I go by the owners manual at least as long as the warranty is in effect. I drive a 2003 Sable that has 180,000 miles on it. As you can surmise they are mostly highway miles. I went with the recommended 5000 miles between changes for the first 35,000 warranteed miles then switched to 100% synthetic oil and 10,000 mile intervals. I believe the long trips and advances in engine and oil technology makes this a perfectly sane practice.