There are several threads on this subject, some pretty extensive.
There are lots more that you can find via the search engine. Some are older, but this kind of thing doesn’t change much over time.
I recently have been using the Nyogel. But for years used either Superlube grease or a “pure” silicon grease. All mentioned in one or more of the links.
As to O-Rings, first you need to find out the specifics of what your light(s) are using. Then read the specs on the lube that you want to use. They typically mention compatibility.
But this should give you lots of info to get started.
