That's hardly anything new, almost every Xterra (or any nissan with the VG33) has had or will have a cracked manifold, or several of them. I doubt it has anything to do with your mileage though... never has impacted my mileage. Already had passenger side replaced and driver's side has a 2 inch crack now. I still get 18 - 20 MPG normally. But one thing is true; you have to replace the manifold as there is no way to repair it and have it hold.