RE: A new but old engine. :)
Coca-cola!
I am not kidding. My brother introduced me to that one. A differential from a 57 Willy's TJ5 Jeep. Seized and rusty. Coke for a day, spray with a hose-pipe, and it was turning again. Gritty, but turning. If I hadn't seen it done, I would have never believed it.
gus