Is there a substitute for plumbers grease?
Use as close to 90%-100% Silicone grease, as you can find. Other types of grease will either degrade the rubber parts or are too thin to be of any use. Danco, an O-ring manufacturer, supplies grease that is specifically made for these types of applications.
First, keep the rod well lubricated. When you dump a tank and the handle is pulled to the open position, simply shoot the stainless rod with a good shot of lubricant spray containing silicon or Teflon.