My 2013 kw with isx15 was using anywhere from 3-4 gallons between oil changes of 30k miles. Shop had it for dot inspection and said front seal was leaking. Took it to local Cummins to have engine fixed @420k miles.
If your engine s/n less than 79812545
You will have valve seal problem, which causes oil burn problem. Cummins started new updated seal production 2015 feb. First thing I would check turbo seals and valve seals.
We had an ISX with the same problem and it would throw the code for the crankcase breather filter sensor on the left side of the filter housing , it would say crackcase pressure was too high bcs combustion was leaking from the cylinders into the block, even tho we had hanged the connector and sensor, did an overhaul, the rings were wasted, and it fixed all its problems no more codes or oil consumption