Re: 15,000 miles between oil changes?
There are a few oil analysis labs that will tell you how well your oil service interval is doing. You send in a sample of your oil and they will tell you your wear metal content, what your oil's additive package strength is (i.e. how much additive life is left), ph, fuel content, etc. From that you can make an educated guess as whether you can go longer bvetween changes or if you've gone long enough.