Lubrication is required any time metal moves against metal. I use Haynes Lubri-Film or Molykote 55, with a preference for the former. Anything more than a thin coat is a waste. It's good to wipe your O-rings down and re-apply on a regular basis. It will increase water resistance and prevent premature wear of your O-rings and threads.
Maglite suggests using plain old petroleum jelly...which I have been using for almost 20 years on my 2D... never had an o ring fail or even degrade using it. I use it on all my lights and never had an issue. YMMV.
I use nyogel, armytek do a tube which should last a long time. Less is more, if possible remove O ring and massage into, couple of dabs on threads. Then work a few times with tail cap and any excess remove.
I used Nyogel for many years and I recently shifted to Super Lube Multi-Purpose Synthetic Grease without any particular knowledge on how they actually compare when it comes to flashlight lubrication. My gut says that the Super Lube "feels" a bit more lubricative whereas the Nyogel "feels" relatively sticky: I observe that discoloration (of the grease) takes longer with the Super Lube and that threading and unthreading action is a bit smoother.
I may have bet on the wrong horse, though. Frankly, I chimed in for provoking some well-informed answers to benefit from! :laughing: