Due to the miniscule amount of fluid used in the PTU, it is best to do frequent fluid changes (every 25-30K miles or so for PTU). The RDU has a bit more fluid, and is usually not as stressed, but still would not go longer than 50K before a change. If you are comfortable with having the car on jackstands, or can rent a bay with a lift, the "hassle" is greatly reduced!