Oil tanks in hydraulic and lubrication systems must provide specific basic functions for proper operation. This includes a suitable filling port for a safe and clean filling of the oil, a qualified air breather to prevent the penetration of any ambient air contaminants in the event of level fluctuations, a tank top return filter to keep the oil clean, liquid level monitoring to control operational readiness, temperature monitoring and a reliable sampling port, so oil samples remain free of secondary contamination.