Chuck something like Soltron in, preferably from new. Kills bugs,
absorbs moisture in suspension - but won't deal with water at tank
bottom, breaks up sludges and residues all to a micron or so so it all
passes harmlessly through filters and is combusted in the engine. I
took 5 galls of the blackest and nastiest red diesel and chucked in
the recommended dose. After a week it was not back to clear bright
light red, but is was clear and red.
In addition any soot deposits in engine - backs of valves etc etc just
brush off with a soft brush.