I had the same question... Here are a couple resources I found to be I of some value---
(California based site on how long you can stretch your oil--- mine is about 7,000 mi on an 02 2.5 OBW)
(gives you the logic behind why you can go longer than manufacturer recommended intervals)
Of course, we should check our oil often and if it looks too funky and has been a long time since the last change--- might as well change it.