I dont own one, but I make my living fixing these things day in and day out (im still a tech assistant, im young, 21 and not done with all my training just yet and waiting on classes to open up from ford in my area) and when I say 10,000-15,000 miles max is because fuel quality is very critical in these trucks. Ive pulled filters on several 08's that with 10-15k on the clock that looked terrible. I have it printed somewhere maybe I could share if I can find it, but the last time we had a ford rep at our dealer he told all the diesel techs that we need to start selling filters and recommending them every 10,000 miles for 6.0's and 6.4's. I would go past it if you know that the fuel you are putting into your truck is top quality, some stations around here sell junk.
Of course they're going to tell you that you need to start selling them at 10k miles. Ford is in the business of making money. Most people DON'T read their owner's manual; maybe the few that come here are the exception, but at 10k miles, you're going to get $200+ in parts and labor for something that will only take around 30 minutes.
The manual for the 6.4L DOES NOT say to change your oil and fuel filters as often as everyone says. It has two different schedules, normal and special operating conditions. I don't remember *exactly* what the manual says, but I got something in the mail from Ford the other day that seems to match what I remember. Here's the gist of what's in that letter I got (and I'm 100% certain it matches my owner's manual).
Oil & Filter for the 6.0L, under normal operating conditions, change at 7500 miles. Under special operating conditions, change every 5k miles or 200 hours.
Oil & Filter for the 6.4L, under normal operating conditions, change every 10k miles; special, change every 5k miles or 200 hours.
Fuel & Filters (both) for the 6.0L, change every 15k miles; under special, change every 10k miles or 400 hours.
Fuel & Filters (both) for the 6.4L, change every 20k miles; special, every 10k miles or 400 hours.
So... translation? It has a lot to do with how the truck is used. I can have 20k miles on my truck before needing to replace the fuel & filters. Why? Because I don't idle, drive in heavy traffic (I can usually keep it above 55 mph), or tow/haul. And even if I did, I'd switch to the special operating conditions. But I sure as heck wouldn't do maintenance any sooner than I had to.
Peace of mind is not only expensive but unnecessary.