As far as the 6.0 engine goes, it's every 8000km for Canadians (we're considered severe use). I know some guys change it by the hour meter at every 200 hrs. The hour meter method is what we use on our equipment at work as idling time is supposed to be taken into account. I change mine every 7500km because it's easier to keep track on the odometer...going to change at 67500,next at 75000, then 82500, 90000 and so on. Not sure what the 6.4 PSD suggests though...can't be much different.
The Hauler - '05 F-350 4x4 CC Dually 6.0 Powerstroke
The Project - '90 Ford Bronco 4x4
The wife's - '11 Ford Escape
For every minute you are away from work...you can add two minutes to your life!