Thetford Drain Valve Lubricant, 20 oz.. Our special water-soluble formula prevents seal wear, sticky drain valves and keeps all the plumbing lines operating smoothly. Approximately 2 ounces in the holding tank effectively lubricates drain valves, lines and walls. Lubricates holding tank systems and drain valves from the inside Restores easy valve operation Coats plumbing lines for smooth tank evacuation. Works in both grey and black water tanks 100% biodegradable liquid