When to change?

Always carry an extra one with you if doing the change by stripe program. It is a good idea anyway.

 

somehow very little amount of soot is going to fuel system. that's why the filter is black.

 

I'd have a hard time putting that fuel filter back in my truck. That looks trashed. You should see what your tanks look like inside.
I'm freaked out. My ISX is nothing like that.

 

Dude you have me freaked out with that fuel filter.
I was going what kind of oil filter is that?

 

the tank will be clear and fuel would not be very black. like I said it's very small amount of soot. Take your truck to Cummins only dealer. Show them old filter. My old 14L series 60 had this issue. The davco filter was getting black like yours. Changed O-rings on injectors and never saw black fuel filter again. I suggest you to do this. Idle the engine at night. Open both fuel tanks and try to see bubbles coming out . Use good flashlight. If you see small bubbles then it's O-rings .

 

That filter looks pretty dirty obviously. I change mine when it gets about 3/4 to the top.
I would always keep an extra filter on your truck.