Hilarious picture, from Chapelle Show. That's more referring to dry ashy lips that are burnt from a crack pipe.
Sounds like an honest mechanic, lucky you. This is the downfall of diesel vehicles for daily drivers with relatively short commutes. Glad you got it fixe, hopefully it was inexpensive for you.
I had an old GM 3.2 motor that had a problem with carbon buildup on oil drain lines somewhere in the engine. Oil couldn't properly drain back and would burn off and I would be short about a quart and a half between oil changes.
The suggested fix by other owners of the same engine was to use Seafoam in the oil before changes and suck up some Seafoam in the brake booster. Something in the Seafoam was good at breaking down the carbon buildup and freeing up those passages. Don't know how it worked... but it worked. No more oil being burned.
Not sure if there is some similar tip for you. Find a Cummins forum and see if anyone has suggestions. Good luck!