In two words...dirty fuel. If it was gooey and slimey, it would probably be fungus. However, this is probably from fuel that has sat for a long time and some of the tars have settled. I had this from a fuel stop on a trip. Fought it for a long time until I ran a bunch of biodiesel through the fuel system and eventually it cleared. Use quality fuel filters and several spare pre-filters.

The best thing that eliminated the majority of it was draining the tank. It is pretty easy to do from the hoses on the bottom of the tank. (Do it when the tank is low on fuel and have several containers on hand. I used a double-barb fitting, hose and locking pliers to pinch off the hose between containers.)

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Sam

84 300SD 350K+ miles ( Blue Belle )