A simple oil analysis will tell you exactly what condition your oil is in. If it is in good shape, don't change it.
I know of one semi tractor that is running on 340,000 miles with only filter changes (including an aftermarket bypass filter) and make-up oil added. That's right, 340,000 miles without an oil change.
Compare that to PM's at every 15,000 miles at $290 each, and a $30 oil analysis (such as Titan labs) pays for itself pretty quick.
