I just thought it would be a good idea to put something about these up here to clarify things, i've noticed occasional confusion in some threads. Most people know, but if you are just starting out then it's something you ought to learn. Vaseline is the trade name of a petroleum jelly, it says so on the tub & there are a number of cheaper alternatives on the market. The words on the tub are a pretty good clue that it is a petroleum based product and before putting it anywhere your O ring you should read the label. Vaseline used to be used for 'packing' in water resistant products which are mainly made out of metal, (and there are still some applications where this is a good method - not photography)you would fill all available space in the component with Vaseline so any water would find it difficult to penetrate. If water did/does penetrate the hope was that the Vaseline would protect the component from corrosion by leaving an oily coating on the metal. Some divers coat their knives in vaseline to protect it against the salt water, i don't because i keep it well away from my diving kit as i don't want it on my O rings even accidentally. Silicone grease is NOT a petroleum product, it's a thickened silicone oil used not to pack, but to lubricate rubbers and synthetic rubbers. It is not used to 'pack' as the products it is designed to lubricate are the correct packing in themselves. O rings are made from rubber or plastic, they are so common today that we take them for granted, but they were a breakthrough in design in their early days. The rubber or synthetic rubber they are made from can be very, very alergic to petroleum products. A cheap piece of rubber it might be, but your camera is at its mercy, treat it with respect. It is a piece of precision engineering designed to fit exactly into the space between the front & back of your housing and does not need any packing to make it fit, it does need a tiny, tiny amount of lubrication. look in any good jeweller's window at cultured pearls, a bead about that size is the maximum you should need on the end of your finger to lubricate your O ring. when applying the lubricant gently rub the O ring between your fingers (i'm assuming you have washed your hands before setting your camera us here), don't pull it along the length as this may stretch it and then it won't fit. The debate as to which silicone grease to use is not one i propose to enter here, if you want to be safe use the one recommended/supplied by the manufacturer. and do NOT use it on the O ring in your cylinder especially if you are using nitrox, but that's a whole different topic.