Cummins ISX CM870 Black diesel fuel and clogging fuel filter

Hello everyone

My dads 2007 Volvo VNL 780 with a Cummins ISX CM870 has been giving him a hard time with clogging of his fuel filter, he has been getting black oily diesel fuel, I opened the fuel filter and it looks like oil, at first we changed out the injector seals but still getting black diesel and clogged fuel filters, then we changed out the fuel pump and that didn’t help we check all the fuel lines and they are good, so we still haven’t found the issue has anyone experienced this type of issue before. Thank you in advance

 

I am going to tell him to run that diesel kleen for a couple of weeks and see what happens