Just wanted to share this tip in case some have not tried this. I decided to pour vegetable oil into black water tanks and work the dump valves to lubricate them. I had forgotten what a difference this makes, valves no longer stick and work better than new!
I've read online that you put in about 1/2 of a quart bottle into each tank, with them empty and the valves shut then work the valves as rapidly as possible a few times, after it sits for 15 minutes to allow the oil time to run down into the valve area. You then add the rest of the oil and allow it to sit for at least 1/2 hour before any water is added to either tank.
I gave up health foods... at my age I need all the preservatives I can get.