My experiance with these products pertains only to automatic transmissions.
What follows is only observation, I have no basis in fact for anything.
These products seem to soften the rubber. This seems to work for a while
but the rubber seems to get softer and softer. This presents two problems
as I see it; 1) every other rubber part in contact with the oil will also
get suitably softer, and 2) the seal will fail in the end anyway, except now
you have a gooy mess to clean up. Personally I'd fix the seal.
je