Your "clay" might be algae that you sucked into your filter from your tank. That happens when you don't flow enough fuel (like your under-powered, over-cylindered chevy's [img]tongue.gif[/img] ).
Seriously, it comes from fuel that sits too long in the storage tank. I would recommend two things for you. First, switch places that you get your fuel. Find a place that goes through diesel quite regularly (i.e. truck stop, Co-op, really popular station). This is why these places are so highly recommended on bulletin boards. It helps avoid these problems because fresh fuel is always in the tanks.
Second, I would begin a biocide treatment regimen. Go to the store and find a good biocide for your truck and use it. If you are worried about the additive issue, ask yourself this. What method of repair are the "technicians" going to use to cure an algae problem? Odds are it will be some kind of biocide. Not to mention, if you don't have an algae problem, the treatment won't hurt.
\'99 2500 ISB QC SLT (No Leather!),4x4,5sp w/McLeod,4:10,BP8x11\'s,B-1,FMS,EZ,PE4200,LPG,H2O/CH4O,4\"exh.,Pac-Brake,OBA,more switches than you,Line-X,V-1,lts,siren,lic. plt. frm says \"<b><i>Diesel Fumes Make Me Horny!</i></b>\",and much more goofy stuff.