I use the bypass filter from:
www.oilguard.com
If you are interested in extended oil change intervals, you really need to do oil analysis to let you know when you NEED to change the oil. So far, I have (with mostly solo daily driver miles) been able to go about 17,000 miles before I need to change the oil.
BTW: This is for the Ford 7.3L diesel. The 6.0 shears the oil too much to extend your oil changes from what I hear. Check the diesel forums for Ford 6.0 for more info on this topic if you have this engine. YMMV.
Eric