Drain Valve Lubrication

So how do you lubricate the drain valves (grey and black tanks)?
My tanks are clean, so now would be a good time to do it, and the slide valves seem to be getting stiffer.

Just clean the sliders with q-tips and add petroleum jelly?
Or is there a special grease?
Or do you tear apart the drain valve first?

Or doesn't Thetford have something that you just pour in the tank? How does that work?

Thanks in advance for any advice.