I'm told that Russia has an alternative means for making diesel that doesn't involve refining crude oil. You can make diesel from cooking oil and coal. The shortage doesn't effect diesel the way it does gasoline.

Could also be high concentrations of ethanol, which (if otherwise contaminated with water and debris) would cause goopy sludge to appear.

Who knows? You could even have different agents at different points in the refining and distribution process each cutting it with "something" and by the time it gets pumped into a car it could have multiple adulterants in it that are reacting with unintended effects.