GLTHFJ60
Stupid is as stupid does
That seems really black for a fuel filter at any mileage. Not sure on the 6.4 but when we had those symptoms on our C6.6 engines the common rail pump lube oil was leaking into the fuel side of the pump and the return fuel would black the filters out. The 6.4 is old enough to have a oil lubricated injection pump unlike the newer all fuel lubed pumps. No idea though about the 6.4 specifically.
Googling I see mention of Asphaltate but I have associated asphaltates with nasty sludge and black filters, get's misdiagnosed as algae often.
If it's asphaltate build up I sugest you ttreat it pretty quick and get spare fuel filters if you don't have them already.
That's a good idea. I think the 6.0 and 6.4 are similar in that they have HPOP to actuate the injectors. Perhaps one of them is leaking, and putting engine oil into the fuel system, that's returned to the tank, then blacks out the filters? Not even sure if that's possible on a 6.4.
That'd be nuts if that was the case. Buddy of mine with a 7.3 had something happen to him, but it was catastrophic, as in all of his oil was pumped into the engine through the broken injector, vs into the fuel return.