The WD is short for Water Displacement. WD40 is not a lubricant. It will clean your O-rings but it will also dry them out if not lubed after cleaning.
I've had the debate of WD-40 with lots of people over the years. That it's a lubricant. Nothing sticks to it because it evaporates.
WD-40 gets used on my fishing lures and baits only.
I get more lubrication rubbing a finger alongside my nose and rubbing it on, then with WD-40.
YMMV
Principal at work: Like dissolves Like.
- Some O-rings are petroleum-based. These are generally black, somewhat firm to fingernail pressure, and not very stretchy.
- Some O-rings are silicone-based. These can be various colored, a bit softer to the fingernail pressure, and a bit more stretchy.
Yeah, there are a whole lot more but for the most part with lights this is what you likely have to deal with.
I know, not very scientific determination, but we are working with cheap lights here where they fabricate performance statistics. Probably most of the time they don’t have a clue what O-ring is bring used and it may change depending on what bulk shipment they got from wherever. I doubt you are ever going to find out.
For threads….metal on metal > use whatever grease or lube you want. Use sparingly, and try not t goop everything up with it.
But, most of the metal threads end in at a O-ring. If you are using the wrong lube, you may end up compromising the O-ring over time.
Not a critical O-ring, not a big deal. You may not even actually need it….so who cares.
I, on the other hand, am a scuba diver. My lights and my cameras are $$$ and those O-ring need to be clean, properly lubed, and taken care of, or I pay the price. Literally.
On petroleum-based O-rings you can use silicone lube.
On silicone-based O-rings you may be able to get away with petroleum lube….but those come in so many different formulations it’s a headache to sort out. I just use a ‘universal lube’ (generally damn expensive; Christo-lube, Tribolube) or one that I KNOW is formulated for silicone, generally less expensive but not terrible (stuff that comes with my camera equipment).
A small amount goes a long way so even an expensive tube is not that big of a deal.
Screw it up or don’t know, some hardware stores have a decent stock of O-rings. Then you know you can use silicone grease.
If it comes out of a spray bottle I wouldn’t trust it.
WD-40 is not a lubricant. It’s a [water displacement] and a poor lube.
For older lights without anodized threads in the tailcap, i find that candle wax does the trick best.
Use the softest candle wax you can find, which will stick to the aluminium and smear rather than crumble.
Unlike any other lubricant, it doesn’t dissolve the aluminium, so you don’t get that awful grey gunk you normally get, even with lumpy silicone grease.
True story. 
WTH!! Hands on experience, HUH!!
Thanks. You’re the person to ask this question I’ve had. At some point, do you guys have to just replace the Orings on a time schedule to maintain the waterproof ratings?
I have lights that I’ve kept the rings in good cond. for long periods of time, but I don’t really know if they’re as water resistant as they used to be. Lots of use can fatigue some materials, especially cheapo stuff.
zen
i tried marine grease on one light and it seems ok so far; will see over time how the o-rings survives
advantage is that it is not water soluble. so will keep water out of the light
desadvantage is that if you get some on fingers, it is difficult to clean…
as for o-ring compatibility with grease types, it will depend on the elastomeric material used, for exemple :
- silicon might not like some greases. silicon is expensive and i would be surprised to find it there, not compatible with oil, but excellent on high and low temps- quick curing too
- acm should be fine - perfect for oil, not good with alcool
- nbr may be ok - cheap , excellent with water and alcool, not good with oil
- epdm may not like
cheapest materialonly good with water
others excellent materials like fkm, viton, hnbr would not be used because of cost
Its a solvent not a lubricant. Clean parts work better than dirty or rusted one’s.
I don’t think anybody even knows whats in this stuff.
Well, I have been using Petroleum Jelly.
No problems so far with any o-rings, but based on what I have read in this thread I believe it’s a good time to invest in something more o-ring friendly 
Anyone tried GT185 ?, it’s like wd40 but smells nicer ,does the same job but leaves a very thin Teflon coating behind ,(a dry lubricant ).
Thanks guys for info. Now figuring out which one to use that is cost prohibitive and easy to acquire.
Well. living in the moist pacific northwest, iffn you decide on the snail/slug option, I can offer volume discounts.
my favorite food at xmass !!
That one of those ‘it depends’ questions. I’ve replaced some of the O-rings in Chinese dive lights I get before I took them on a dive. I’ve had other O-rings that I’ve used for years and they seem to be fine.
Generally O-rings are [static=no movement] or [dynamic=movement]. Static can last for years. Their most common problem is permanent deformation if they end up being kind of a “sealing washer/smashed”. That’s not hard to see and those get replaced.
Most of the ones in lights are [static], get compressed to seal, but don’t get smashed. With PROPER lube (non-destructive) those can last for a really long time, IF they are decent material in the first place. Not all O-rings are created equal and some of the ones I’ve gotten on cheap lights are kind of suspicious. But, water-resistant to rain and such is simply not the same as 60 feet of water pressure.
Dynamic O-rings are under constant movement (hydraulics, some parts in dive regulators). Those are subject to pressure and wear. In regulators they are recommended to be replaced annually. But, a LOT of that is liability, and pure fear driven capitalism. I’ve overhauled 20 year old regulators, simply cleaned and lubed what was there, replaced nothing, and bet my life on it by diving it. I absolutely test them for performance and leaks, but more often than not they work fine.
Camera case manufactures recommend annual replacement of their O-rings. But, it is time consuming to send them back, and quite costly. OTOH, a failure of one can immediately destroy $$$$ worth of electronics. But, the harsh reality is it’s usually NOT an O-ring failure, it’s something the operator did wrong. So, I’ve overhauled cases myself, but I’ve never sent one it. I absolutely try to keep the compression off if possible when not being used.
Long way of saying, keep them clean, use the right lube, and they’ll last for years.
Permatex Dielectric Grease (made for spark plug boots and battery terminals) has worked for me for years as a budget silicone grease and it’s never caused harm to any of my o-rings, but as stated who knows what they are made of.
I got some Oatey silicone plumbers grease which is supposed to be almost pure but it’s not thick enough for me.
I’ve used Slip 2000 EWG (grease) in a pinch.
BTW, i’ve put different types of o-rings in seperate ziploc bags, saturated each with these lubes and left them for 2+ years (still in there actually), and cant tell a difference between them and the control samples stored in the open at room temp. The only o-rings i have in the bags that i am certain of the type is Viton for what it’s worth. The others…who knows what they’re made of.
Tidbit: The dielectric grease is also good for light bulb sockets, super thin layer on the bulb threads helps prevent seizing :+1: !
Get some Dow Corning high vacuum grease. It is made for this type of stuff. I put some in my Zephal HP bicycle pump and it went from 120 psi to more than 160 PSI. I actually cracked a rim testing the limits of the pump. I can’t see there is anything in a flashlight that is not in the pump. That is aluminum and O rings. About $20 for a half ounce tube.
Electricity
Some lights have bare threads through which the current must flow.
Dielectric Grease all the way! It waterproofs and insulates electrical connections. It’s safe for the O-rings too.
On CPF, I had read that Mobil 1 synthetic red grease in the grease gun tube worked well. I bought some for my lights and for repacking bicycle bearings and it’s worked very well. It has a strange smell but once the light is screwed together, it wasn’t noticeable.