871 oil mixing with fuel

[joed2323]

How's it going guys... I got a buddy that has a 09 Kenworth with the Cummins motor. It has roughly 274046 miles and 18786 hours on the motor. This truck works in the oil fields and has lots of idle/pto time

His primary fuel filter is basically black, you can shine a light into the fuel tank and can not see the bottom of the tank, it definitely looks like oil is getting into the fuel system... And ideas how and what could be the cause? He removed the primary filter and drained the fuel into a oil pan, and can see plain as day that their was quite a bit oil in the catch pan

It is also has been burning some oil as well, he said roughly a quart a week give or take some...
The truck definitely likes to "roll coal" when it's been idling for some time, once you get going through the gears she does leave a grayish/black cloud of smoke pretty much anytime he goes to accelerate through the gears. Seems to smoke at random also

Apparently he recently changed out the dpf as well, thinking it would cut down on his smoke with no luck....

He is currently thinking to do a inframe before it's too late. What are your thoughts on this? I'm sure this is a common issue with these motors and with the amount of hours on this motor.

Could it be as simple as an injector o-ring leaking and mixing oil into the fuel return? Or possibly the high pressure fuel pump mixing with oil?

It was recently at Kenworth for ECM updates and the dpf, and had them check it over for the smoking issue and they said their was nothing wrong with the motor. It also had a tune-up a few months ago
Thanks guys