EGR valve, lots of soot to turn into ash = lots of regeneration time which eats fuel.

an interesting side note on the early 6.7's (7-12). Lots of people bought these truck and when used in light duty driving (no load) it wasn't uncommon for the "change oil" indicator to come on early.... like 3/3.5K miles. Well....no diesel guy is going to let some light come on and tell him to change oil, "my old Cummins I went 7k on oil changes"... so on they would drive. When the system goes into regen, there is an injection event on the exhaust stroke to heat up the cats and dpf. Of course, this process also gets fuel past the rings (thus the jet black oil on these trucks) and the ecm calculates through drive cycles etc that when the crankcase becomes 2%-3% diluted, turn the "change oil" light on. So now our oil level has become WAY overfull, hits the crankshaft, coats the cylinder walls with thin diluted oil, gets past the rings, gets burned, creates more soot, goes into regen more, used more fuel..... and the snowball from soot he77 rolls hard.

I've honestly found many of these trucks "unfixable" based solely on driving habits and use. "IF" someone bought one of these trucks, and worked the L out of it, chances are I wouldn't see them except for service.