A word of warning to anyone considering using the little tubs of Xtar lubricant.
I contacted Xtar to clarify whether the lubricant is silicone grease. I was told it is petroleum based vaseline.
Guaranteed to dissolve O rings.
+1 I bought the same, it's a good product.
since it's sticky and still a "fluid" (well, ..) i *really* like it for sealing metal threads or to water-proof rubber o-rings.
it's a 100% nogo for application on twisty threads, and for example a cauchemar on twisty bare aluminum threads (Quark tactical UI).
Hi all...new to forum. Not sure if this has been covered, but a good, common and inexpensive flashlight thread/O-ring lube is available at most hardware-type stores as "plumbers grease" (really just silicone grease).
Examples:
http://www.lowes.com/pd_817-1409-GR2V-D_?PL=1&productId=3350380
Aloha and welcome to BLF wylde21!
Really?? You sure, Torchy? Vaseline is pretty much inert, safe to use literally on a baby's face. I can't imagine what material it's likely to disolve :\
I have used it for various applications myself, but I am told by divers that it is really bad for O rings.
Hmm. Well I guess they should know, but it's a new one on me.
EDIT
And apparently on some of these guys: http://www.candlepowerforums.com/vb/showthread.php?54243-Safe-to-lubricate-O-rings-with-Petroleum-Jelly, but there seems some concensus against petroleum jelly. Thanks for the tip.
Welcome to the club, wylde21.
Oh sweet! I'm hoppin over to HomeDepot tomorrow to pick up some plumbers greese. Two ounces of this stuff will go along way....lubing flashlight threads.
My Skyray King is a SCREECHY sum-umma-bish. 
Looks almost the same as the Xtar product… are you sure it is different?
Well that depends on what the o rings are made of. Silicone, neoprene or viton will be fine, but natural rubber will be killed by petroleum jelly (Vaseline). Most o rings we see in torches are silicone or neoprene.
Maglite recommends vaseline on their lights. If it’s good enough for maglite and a baby’s face… then it’s good enough for me! 
LOL… Actually I use Super-Lube synthetic grease on my lights, it works well and has teflon. Plumbers grease is good if you want a water proof light, but I’m not sure about the actual lubricating properties of it. You might wind up with worn threads after a while.
High quality silicon grease can be purchased at any camera or SCUBA dive shop cheap.
I use silicone oil for lubing the threads and oring (the one used for diving…)
Maybe the divers don’t like vaseline on o rings because it poses an explosion risk in the presence of high pressure air. (If some one mistakenly used it on the first stage of a regulator for instance).
That would be a serious problem with NITROX, but then you should never lubricate orings on a first stage. They are maintenance free and replaced annually or when they rip. We dont use petroleum based seal lubricants because they can attack some seals & materials.
I don’t know if this is a good product to use or not., but I have been using dielectric tune up grease on the threads and o-rings of my flashlights.
I had it in my tool box and it is clear and clean. I figured that if it is okay for the spark plug boots, it should be okay for the o-rings on a flashlight.
I use “Finish Line Extreme Fluoro” fluorinated grease for my knife pivots and flashlights.
I have an eye on the finish line chain lube, its liquid wax.
I had a knife lubricant, called lightning wax bought ten years ago. there are some finish line products which seem equivalent. Never thought of using this for threads but maybe for anodized threads ok…
For the o rings I have the nextorch from DX, for me it could be a bit less sticky.
For threads I use a special electrical oil(Kontakt 61 ), that makes them much more smooth.