That is the hydraulic oil for your hydraulic system. Where are you from? I'm very familiar with the 3L and know almost everything. You ask me anything. You dont need to refill that because i know for sure you didn't checked it well. You have to unplug the connector from the pump and then loosen the 'black round thingy' en then you wil see the oil rising. This is because all of the oil is in the hydraulic system when it's on pressure.