I use a small plastic "jar" of pure silicone grease. It has the following information on the "jar": Sport Divers Mfg., "O" Ring Lubricant, 1 1/4 oz, and cost $6.50 years ago. I bought it at a Scuba Diving Shop.
Although I have read instructions which suggest treating "O" rings with Vaseline, I agree with Walt that it is not a good idea. It is certainly harmful to latex, one reason to use K-Y jelly on such products. Brian, K-Y jelly will not hurt your "O" rings, but since since K-Y jelly is water soluable, I would not recomend it for that use as it might dissolve and allow water to penetrate your flashlight.
OK, I managed to find some instructions to my Solltaire Mag Light. They state: "Periodic Maintenance: To keep the O-ring seals from drying out and the screw threads operating smoothly, apply several drops of petroleum oil or jelly two or three times a year." Can anyone explain why they would state this?
My Silicone grease was an expensive purchase for the size, but several years later, I still have about 2/3 of it left, say 3/4 of an ounce. I know it is safe for any type of rubber and presumably for most contact since it is, at least as I recall, used on regulator parts that you breathe through.