.....I too thought it was stubborn glycerin which is slowly dropping out, but if I remember correctly it wasn't water soluble......
I don't think it is glycerine. The clue is in your statement "wasn't water soluble" Pure glycerol is hydroscopic and completely miscible with water. My own view is that we are looking at oxidation products such as peroxides and polymers. I have seen this red/brown deposit in the bottom of cubies that have just a few ml of bio after standing for some time. The conditions in you vehicle are pretty harsh if you think about it. Fuel is constantly being circulated through the fuel system and back to the tank via the spill lines. So, it constantly being heated and cooled and exposed to the air.
I'll check solubility for sure tonight, I have some samples of the stuff I've collected, it's been a while since I played with this stuff.
