advice on gas or diesel, etc.
I owned a 2001 f250 cc lb 4x4 5.4l gas with 3.73 limited slip diff and pulling 7k 6 days a week got 8mpg
Now i own a 2004 f350 srw cc lb 4x4 6.0 with 3.73 limited slip diff pulling 7k 6 days a week and now getting 14 mpg in the hills of sc pa.
Lastly my buddy's truck which i got to borrow is a 2002 f250 cc sb 4x4 with 4.10 limited slip and pulling 7k got 9 mpg.
So it is more than just the engine you need to consider 4.10 will pull easier but uses more fuel because engine rpms are higher
Your 08 has crappy mileage because of the dpf filter because someone decided that cutting soot by 50% by burning 50% more fuel makes sense.
Do it better than the last Guy!