Just to let you know... that engine has been apart before, well at least the oil pans were off. Nissan nor any other shop I know uses blue silicon to seal oil pans. So hope you got a warranty. Possibly the engine blew the HG or what not. Pull thermostat first and see how it looks, if its not sludgy and completely clean then more than likely the engine was refreshed a while back. But it has been opened for sure. And bottom end opening is not a good thing when an engine is toegether in a junkyard where the car was not wrecked. I always look for the wrecked cars with engines still in them, at least then I know it was running good enough to run 40 MPH into a wall.