Cummins ISX (non Egr) Carbon in oil found each time (CAT lab anaysis)

Heya Guys.

Each time i do oil analysis they warn me that i have carbon in to oil , i do my tests at the Caterpillar Lab so is the good stuff.
I call Cummins and they told me is the blow-by and they suggested dyno ,.. so i did the dyno test and the blow-by is ok . Next they told me that air admission problems will cause this . Somebody have an idea ?
BtW : it's a Cummins ISX 500 , 2001 Non Egr and the oil test is done always at 15000 miles

Thx ,

 

Thank you all , ISX oil pressure is 45 PSI flat @ 1300RPM (my cruise speed) , the rest of the stuff in the oil analysis look really good .
I run a mix off 6 gall of Rotella synthetic + 5 gall rotella regular on my oil change.
The Air-to-Air Rad is original (2001) and i have no air leak ,... in time maybe there is some air obstruction build inside the CAC, who knows.
B.T.W I change my air filter each 50k miles .