I recently discovered a tank with 50 liters of diesel that is probably
more than 15 years, old from what I know of the boat history.

The 'diesel' or liquid is like syrup, does not smell diesel but varsish.

What is the reaction that can alter diesel ?
Can this liquid be used in the engine ? in a heater ?
Do I need to rince the tank before use ?

TIA, I am a newbie on this forum so sorry if I am OT. The boat is a
barge built in 1929