There is rarely a reason to change synthetic oil sooner than recommended. More frequent oil changes are a waste of money. I know the old school rules for non-synthetic changers were every 3k depending on driving conditions, but those rules aren't the same for synthetic oil.
That being said, and yes I know I'm contradicting myself a bit, but while the oil can go 15k miles, I'm not convinced that the filters can go that long. Does anyone have any opinions on filter life? I've never had a filter fail, and my last 4 cars have all had synthetic oil (Mobil 1) in the X5's and Corvettes, so the filters are probably good for that length of time as well. (assuming you don't hit something in the road and tear the thing off the bottom of the car, but hey, who would have done that in the last month or so?
