300,000 miles shouldn't be a problem and you could probably go much further with good maintenance. I had the heads off my 3.0 just before it hit 200,000. There was not only no ridge at the top of the cylinders, but I could still see cross hatch marks on the cylinder walls. I hear that the steel used in the Mitsu engines is pretty good. I may end up taking mine to near 300K if it works out.