After 500,000+ miles, you may experience lower oil pressure due to O-rings located on the oil suction manifold hardening and becoming unable to maintain a proper seal. When those O-rings lose their seal and their suction strength, they need to be replaced by dropping the oil pan and replacing all of them.
The common misconception is that the oil pump fails at this mile marker; therefore, many shops suggest replacing the pump when it is really just the O-rings that have gone bad. In fact, in the past, some shops have rebuilt entire engines when it was completely unnecessary. Let us know whenever you are experiencing low oil pressure with your DD15.