SuperLube. It’s like $8 for a tube and that’s good for a long time plus use it for other stuff too! Works great in my ratchets and bearings. Plus it’s not really toxic.
It may not matter for flashlights, but silicone lubricants and greases are not allowed in the building or anywhere near telescope lens or reflectors, glass or metal. Silicone will outgas and coat the surface with an impossible-to-remove film. The film prevents the addition of desired optical surface coatings and finishes, and destroys the performance of the optic.
That said, Dow Corning makes good silicone greases for vacuum chamber sealing with both conductive and dielectric varieties. But if the threads of the flashlight are used to carry current then a dielectric grease would not be recommended. The internet has lots of pictures of melted and burnt connectors, damaged alternators and regulators on motorcycle forums where folks filled the connectors full of dielectric grease to keep out moisture.
One good use of dielectric grease was on the porcelain insulator on spark plugs. Keeps moisture and dirt out, and lubricates the boot. Haven’t had to change plugs in such a long time they never wear out.
I just put Tef-Gel you sent me on a Thrunite T10T. I agree it is very sticky. It does a very good job of preventing me from feeling the titanum threads grinding on each other. It also makes the O ring slide smoothly.
I agree it is thick and makes unscrewing the light a bit more slow, and resistant than thinner lubricants.
I find Munky Spunk more slippery, but also thinner. I have put it on another T10T and I can feel the titanium thread grittyness, and it is less resistant to unscrewing, as it makes the O ring slide more easily.
Remains to be seen how well the TefGel maintains its viscosity, I will not be happy if it becomes dryer and resists unscrewing any more than it does. But I respect the comment that the material does not dry out.
My initial impressions is
TefGel is for sake of example, a 90 weight lubricant, while the Munky Spunk is a 50 weight lubricant. Those are not factual values, just my attempt to give a relative density comparison.
I do think Tef-Gel is a viable, inexpensive option for threads and O rings.
I like Munky Spunk a bit better for O rings, but a bit less for titanium threads.
Another lube I find works well on O rings is Nano Oil, but it is too thin to prevent Ti threads from grinding on each other. And Nano Oil evaporates, so though that is good for depositing micro lube on metal, it is not stable on O rings, and needs reapplying periodically.
Superlube silicone grease is cheap, relatively easy to find local, and has PFTE in it, alternatively a tube of dielectric grease from an auto parts store would do similarly well but without the PFTE additive.
I’m fond of Krytox greases, particularly on titanium, but it is prohibitively expensive to buy just for a light or two.
If you visit the mechanical keyboard community you can get a few grades of Krytox for as low as 5gm. Though I don’t think you will get the titanium variants