fwiw, I feel there are some seal conditioner additives and detergent additives than can be of use in some circumstances, not that you can't get both of those things pre-mixed with certain oils (taking any potential "compatibility " issues out of the equation) in one form or another.

Something that's actually designed to "stop" or plug a leak on the other hand, coolant or oil, I'd try to avoid unless it's a last ditch effort and you really don't care about the car - though I know of people who've used radiator stop leak with success.