Diesel

[NorthernRaider]

cloudy / milky or black slimy bits is anaerobic bacteria which can clog fuel filters, a drop of gasoline or acetone in the jerry can should help the cetane level. or you can simply blend it 50 n/50 with fresh diesel. crap diesel may also show signs of lacquering eg a hard varnish like coating on the sides of the fuel can.

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Fuel stored in sealed containers like good quality jerry cans ( with NATO spec coated insides) should not be able to absorb moisture or air so any growth will be anaerobic and less messy than aerobic fungus mould, diesel is not as refined as petrol and has less volatile organic compounds to settle out or evaporate, I would just mix it with fresh diesel.