Oil in diesel fuel filter?

Algae. If you look inside of your fuel tanks, you will see it laying on the bottom. There are several products that will help clean it up, but you will be changing filters often. Th best way iIfound to clean the tanks up, is to drain and remove the tanks. Clean the insides out with a HEATED pressure washer. This is how I got mine clean. After that, keep up with the additives to prevent reoccurrence.

 

To say the least, the stuff is a mess, but is is manageable .

 

Not to hijack, But, I would guess this must be in the tank and filters of every Firetruck I have ever used.
but have never had a mechanic say anything about it. are they just making money on repair issues as we send trucks out for stuff/ Maint.?? heck one truck is 18yrs old and we don't put any additives in tank, just fuel card and go. What causes this algae?

 

not that I ever noticed or have been told... been in fire service 30years...
we run them short trips and really hard ( they are garaged indoors with heat in the bays, but like 4 degrees out and truck is started guys hop on and out the door we go usually at a good clip) and they get put up wet... for the last few trucks we have had to deal with DEF and regens, but I am of the understanding that Fire trucks will be exempt from all that nonsense.... never good when you are pumping at a fire and the truck wants to regen. and drops out of pump gear

 

One reason for algae is the reduction of sulphur. Another contributor is biodiesel. Biodiesel is made up of non-petroleum products. When biodiesel gets heated in the engine and sent back to the tank through the return line, it warms up all of fuel in the tank. Summer sun heats it up faster. Give it a chance to grow and it will. The longer biodiesel sits, the more it grows.
ClubberLang, I do not know what fuel your fire trucks are using, but I would guess you have been fortunate enough to not have used biodiesel.
I bought a Peterbilt that was not ran for about to years and that's how I started to deal with it. When I took the filters off and dumped them out, it oozed out in a glob. The engine would starve out and quit running. The pick up tube in the tanks had been plugged up with algae, so I removed the suction line at the filter base and blew compressed air into it and it cleared it.
I did several searches on this site and Googled it as well. There is a lot of information out there on the problems and products with algae.
My dad used to do service on firetrucks. They stay nice and clean, but like you said, they get rode hard and put away wet.