Rear main seals leaking on these trucks is a very, very rare problem. Most of the time it is a O ring on the HPO lines that is leaking or the CCV saturating the intake boots, which drip into the valley, which drips down the back of the block. This is why the CCV mod should be done on all Powerstrokes. Our injectors function on oil pressure so it is kind of like a saftey feature, if you loose enough oil the truck will not run before any damage can happen. I have seen these trucks 2 gal low and still run fine. Most find that if they fill to the top of the hash marks the truck will burn a quart or two off, but if you fill to about 1/2 way in the marks it won't burn any oil, I have found this to be true on my truck. Fill your truck by the dipstick not by the number of quarts. BTW, FYI Ford considers anything under 1 quart in a 1000 miles is perfectly normal, I find 1 quart every 1000 miles a bit excessive, but this is what Ford says!!
As far as intervals goes everyone has their comfort level, but to be perfectly honest you are probably throwing away perfectly good oil. I highly recommend a Blackstone oil analysis be done frequently, especially after just buying a used vehicle. It is amazing what they can tell you about your engine, and how well your air filtration is working. They can warn you of a problem long before it becomes a major failure. I send off a sample about every other or third oil change. I change at 5K and every time I get my analysis back they encourage me to go to a 7500 mile interval as the oil still shows much life in it. I just feel better doing it at 5K but it is good to know that if I go over it's not going to hurt anything,
NCHornet