1 kg diesel oil is how many liters?
Volume = (mass)/(density) The density of diesel oil is about 0.85 kg per litre. Your 1 kg divided by 0.85 kg/litre equals about 1.176 litres.
There are many variables here as the temperature and type of diesel effects the mass. However, Diesel fuel oil 20 to 60 at 15ºC has a density of 0.820 to 0.950 kg/L I think a reasonably good average value is Diesel oil 40 at 15ºC with a density of 0.850 kg/L 1 metric ton = 1000 kg therefore 1000 kg / 0.850 kg/= 1176.4705 liters
That depends what substance the 2000 liters are full of. 2000 liters of air have very few kg of mass. 2000 liters of water have 2000 kg of mass. 2000 liters of ice have less mass than the 2000 liters of water. 2000 liters of lead, gold, or rock have many many kg of mass. 2000 liters of empty volume have no mass at all.
There are many different grades of hydraulic oil, you would have to ask the manufacturer of the particular type you are using. A rough ball park figure would be 0.88 kg/liter, so 1.14 liters/kg. Why not weigh a liter of it, you could do that on kitchen scales, but take off the weight of the container.