Lots of discussions here about lubricants. Search these forums using the words "grease" or "lubricants" and open the threads with 3 or more pages.
I think the key to lubing the o-ring under the lens (which I totally agree with, by the way) is patience and cleaning your hands with soap and water halfway through the procedure.
As stated, take the lens assembly apart, roll the o-ring in lube in your fingers so just a fine coating remains. Carefully place the o-ring into position inside the lens housing. Then wipe your hands, carefully wipe down the outside of the light to remove all traces of lube that might have gotten on there ... then carefully wash your hands with a powerful soap or mild degreaser to remove all traces of lube from your skin.
Now, make sure the lens is 100% clean and carefully drop it into place.
Screw the bezel back on and you're all set.