Yes and no.
I have often tried the additive (some of the American ones although more expensive seem best).
A couple of old vehicles stopped leaking within a few days driving never to be a problem again. Others it did SFA. If the seal is shrunk from age or a smooth groove worn in the shaft - good chance. If it is torn broken or rough bearing surface - no good. You will not know until you try.
I think if you have a fairly new vehicle it is not going to work as the seal is more likely to be damaged than shrunk or a groove worn in the shaft as it works by swelling the seal and giving it back its flexibility.
In your case I would certainly give it a go. It is not going to hurt anything and $30 might save many hours of labour pulling gearboxes. Give it time, might take 1,000km.
Lang
This one has definitely worked on a couple of army jeeps and miraculously on a 1915 Willys Overland.