My fuel water separator filter was nasty at only 6k miles

[Redland1]

So I flipped over the fuel water separator into a cut open gallon water bottle and the nasty stuff that came out has me wondering what the heck is going on that I have all that gunk in there.

[Rawze]

check your fuel tanks. Maybe getting full of algae?.

[JMBT]

Yep, look in the tank filler neck with a really bright flashlight when you tanks are low on fuel.. nearly every truck I have ever done this on has had shi#t in the bottom of the tanks

[walterk]

I had mine much worse than that when I bought my truck. Most of the time the service guys just change the filter and off you go... Only, when you do it yourself you can clean the whole thing properly. I drain all fuel out and clean the bottom every second time. The fuel in your tanks in not that clean and the black stuff on your filter is a clear proof of this.
This is a good read on fuel and additives:
https://rawze.com/forums/showthread.php?tid=112