The negative excess pressure created in the intake stroke of the displacement units sucks up the oil automatically.
This suction capacity is highly dependent on the construction design of the pump, its geometric displacement and rotation speed (large pumps must run slower to still be able to suck up the oil themselves).
If larger rotation speeds are required, the oil must be fed under pressure. Supply pumpsare used for this, which are used in open or closed circuits (however, here they can be kept significantly smaller than "flushing pumps").