The MQL devices are each connected to the tank extension with a standard hose line and a solenoid valve. The plant control unit monitors the fill levels of the MQL units and triggers a refilling process if required. For this purpose, the corresponding solenoid valve in the refill line is opened so that the lubricant, which is under excess pressure, is conveyed from the reservoir to the MQL unit. The refilling process does not affect ongoing operation. A tank extension can supply devices of different model families, as long as the same lubricant is used.