Thetford - Drain Valve Lubricant - 24 oz. - 15843. Here's an ounce of prevention in a 24-ounce bottle. Our special formula prevents seal wear, sticky drain valves and keeps all the plumbing lines operating smoothly.. Features:. Prevents sticky valves. Restores operation. Coats plumbing lines for smooth tank outflow. Works in both grey and black water tanks. Lubricating additive