Soft plug, core plug, ?

[RTHarvey]

I originally posted this in the S10 forum, but no response, so I thought I would try it here, which is where I normally post (as I have a 67 C10). I fugure core plugs are about the same on any cast iron block:

One of the soft plugs (or freeze plugs or?) is leaking on my grand daughter's 1992 S10, V6. I think the manufacturer says that no sealant is needed, but that makes me a little nervous. Does anyone have advice on how to do this? Sealant or no? What kind of sealant? Brass or steel? Thanks in advance for any advice.