At the same time, the oil temperature in the same screw-in unit is measured by means of a temperature sensor. The device is not ready for operation until a specific operating temperature is reached. This point in time is shown by means of continuous lit green LEDs. Now, if the filter capacity continues to decrease during the operation of the system due to exhaustion of the filter element, the dynamic pressure in the filter slowly increases. When there is only approx. 25% residual capacity left, four yellow LEDs are also continuously lit. If the capacity is completely exhausted, there are also four red LEDs. In addition to the LEDs, semiconductor switching outputs are activated, which transmit the actual state to machine monitoring. As a result of the very direct and precise pressure measurement, it is possible to move the switch points closer to the opening point of the bypass valve. Overall, this leads to a greater filter capacity being available, which additionally contributes to an improvement of operating costs. In order to signalise the actual state of the filter element to the user at all times, i.e. even when there is no oil being returned to the filter (there is no dynamic pressure at this moment), the yellow and red LEDs and the switching outputs remain activated until the current supply is interrupted or until the reset button is pressed. At the latest, this takes place when the filter element is replaced. This device is self-monitoring; in the event of a malfunction, this is shown by blinking red LEDs and switching output two is activated without activating the yellow LED. Upon customer request, an alarm can be generated after the adjustment of all parameters, if the system is being started without a filter element.