I just bought a 2006 144000 miles and was seeing 22-23 mpg. After changing the oil I am seeing 18-20mpg. I am thinking I removed synthetic oil and replaced it with conventional oil.
What I am asking is has anyone else seen 2-5 mpg gain with syn. oil?

 

Possible, suggest running Amsoil 15w40 marine.

 

I am using Mobile, the same oil I used in my last cummins diesel. A 1998 12v 5spd, 4x4 quad cab which I put 286000 mils with no issues. and 20 plus mpg all the time.

 

I ran Amsoil 15-40 marine and unfortunately saw no appreciable increase in mpg.

 

19-20 is what I get out of my 06 with synthetic Royal Purple 15-40, mine is a 6-speed manual with 4x4.

Is your 06 a 2x4 or 4x4?

 

2006 auto 4x4 dually. the over head is off by 5-6 mpg, showing 5-6 mpg over actual mpg done on paper.

 

That's amazing. We've got the equivalent of an auto with 4.10's with our 0.79 final drive. I got 14.4 mpg with camper,285-75-17 AT2's and Smarty sw#7 default which has been my best tune economy wise.
When I received mine from Dave Smith the lie o meter read 14.1 mpg empty with stock pizza cutters.
I think it turned over 2500 rpm @ 65 mph.

 

I have 3.73 with auto. I have not performed a top end yet. with the 98 12v when I did the top end I would gain 1.5 to 2 mpg. I would do the top end every 50k. I do not think a top end has been done to this truck since new. it does not appear to have valve cover removed.