Here is my system for lubricating both sides of the blades and the groove of both of my drain valves including the toilet flush valve. Been doing this for 19 years, still have the original valves.
First drain and flush both tanks to flush out all solid matter. Close both dump valves.
I have a 3 inch termination cap that has a garden hose bib, attach a small plastic garden hose ball valve to the hose bib to act as a control valve, install the cap on the end of the RV drain pipe with the hose bib/control valve at the lowest position. Shut the control valve. Attach short piece of drain hose/bucket if desired. I just let it drain onto the gravel next to my RV concrete parking slab.
Put about 5-8 gal of water in each tank (just make sure that you have more than 3 inches in the black tank and more than 1 1/2 inches in the grey tank). Put about 1/2 pint of mineral oil in each tank. (about $2.50/pt at Wally World) When you put it down the toilet and opening the flush valve slowly it will lubricate the toilet flush valve blade, when you put it down a sink/shower drain with a "P" trap use a lot of water to flush it out of the "P trap.
Open the black tank drain valve about 1/2 way. This will fill the cavity between the drain valve and the termination cap. Open the control valve a small amount, you want a stream of water smaller than pencil diameter. Remember that the oil will float on the top of the water. As the water slowly drains out occasionally cycle the drain valve open/close, leave the drain valve about 1/2 open until the next time to cycle it. As the oil level interface goes down this will ensure that both sides of the blade and the groove are coated with oil. I have been always been able to feel the ease at wich the valve operates when the oil is coating the blade.
Repeat doing the gray tank drain valve.
2005 Dodge, 2500, auto, 2wd. Pac brake PRXB. CB, Max Brake Brake Controller, Rhino Lining, Aero 60 gal. replacement tank.
1998 Hitchhiker ll, mod#31RLBGBW, RBW L'tl Rocker Hitch, Generac NP50 G generator