For the brains trust. I have a brown viscous liquid that I drain out of my fuel filter from time to time. This sludge does not mix with the diesel or water. Diesel sits on top then the sludge and water underneath.

Any thoughts as to what this may be?

Scrubby

My guess is rust particles ? Black is the usual tank muck, what are your tanks made of ?

Do you use any fuel treatment or conditioner ?

Not rust. Tanks are fibreglass. I do use fuel treatments and I did consider it may be a component of the treatment but I thought the treatment was supposed to be miscible in hydrocarbon.

Sounds to me like it is just a emulsion of fuel and water suspending particles.