Well, opinions vary...
First thing I do to all my Nitecore PD lights is disassembling and cleaning that stinky, sticky blue poop off everywhere. I don't even try if and how the light works, it'll work better afterwards! Nitecore smears that disgusting stuff all around the piston, mostly there's even some on top, where the piston meets the body and it makes the piston sticky (old version only).
While there are surely other good lubes around, I use Nyogel 760 on the EX10/D10. The piston doesn't need any lube at all, just put a small amount on the o-ring, put the piston in and let it work a few times, take it out again and relube, carfully wipe off everything that could have gotten on the piston itself, put it back in and you're done.
What I also did to all my EX10/D10 was taking the brass-rings off the light engines, clean them and the contact surface with Deoxyd Red and treat them with Deoxyd Gold. Some brass-rings had big amounts of black dirt all around, without the lights being old and used a lot. You may postpone this operation to the eventual showing up of misbehaviour though, but think about it when your light ignores clicks, doesn't ramp as it should and so on.
I like the Nitecore PD lights, but they have to be cherished to work properly. If you carry them around in a trousers pocket, the piston risks to catch up lint quite quickly and cleaning/relubing will be necessary more often, you'll notice if piston action gets stiffer. The new version could be less affected by dirt however.
Last but not least, if your piston's too stiff and/or if you're using the longer 14500, you may want to cut off the outer ring of the spring in the piston a centimeter or so. The new versions seem to have less of those problems also.
Have fun with your D10! :wave:
