For my limited torch use, I use light gun oil or nothing at all and Nextorch silicone grease for the sealing rings.
The rings should provide enough waterproofing and any thick grease just collects dust. Also when you change the batteries everything gets covered in grease...
A lot also depends on the torch, that is whether it has sealing rings everywhere, including behind the front glass, whether it is a "twisty" or clicky and how sealed the switches are, as well as the use (rain, swimming, diving..)
Proper waterproof torches will have a magnetic switch so that no water can go in through the switch.