I think I would change out the tensioner before spraying anything on it. First if the tensioner is starting to "go south" spraying anything on it is going to act like cutting oil and speed up the wear process. It's not worth the risk of having it seize or "grenade" on you, if it does you'll have a major $$$ bill on your hands (belts wrapped around crap, overheating, broken fan schroud, - ugly possibilities)