Remember the Tin Man? Well, you should.
Nano-Oil™ is a proprietary formula designed specifically for metal-to-metal contact applications. It works incredibly well on both flashlight threads and o-rings. For the average user, this small 8cc bottle is probably enough for a lifetime.
This should be enough replacement o-rings to last for many years. Daily users might need to replace the o-rings once a year and casual users, every 2-3 years. The lens o-ring should not need replacement unless the glass breaks or you decide two change out your own LED.
Flashlights require regular maintenance if you want them to last and perform at their best.
It's quite simple, but very important. The main task is to lubricate the threads and O-rings. If the threads ever feel "gritty" then those will need to be cleaned with a soft cloth and an evaporating and degreasing cleaner like Windex or Simple Green. The o-rings should get a drop or two of oil if they ever start to feel "stiff."
You'd be surprised by how many different types of oils and greases are out there. You might also be surprised buy how important they are to flashlights. The top contenders for lubricating threads and o-rings are: DuPont Krytox, Super Lube, NyoGel, and Nano-Oil. There is a huge amount of debate on which is the king of the hill. This is one of those forum topics that gets debated endlessly.
I'm an applied research type of person, so I decided first hand experimentation was better than third party hearsay. I bought some of each, tried them all, and to cut a long story short, Nano-Oil gets my vote. It just plain works. Greases have really great "feel" but tend to attract a lot of dirt and get all over everything when you take the light apart.