My brother an I currently own a couple of 2005 CX613's that were formerly Air Product fleet trucks. From what I can tell they have been well maintained through their career at AP. The trucks came to us with 450K and 550K on the odometers. We have been going through the trucks over the last year and a half replacing hoses, EGR's, compressors, turbo's, and other items that needed freshening up. My question is when should we be looking at the internals? I've heard of them needing a "bottom end" at around 750K . Is this necessary? Can a oil sample determine if this needs to be done? Mine now has 650K on it and I run AMSOIL in the truck. It has been running smoothly, and I don't want to spend a bunch of money on a repair that is not needed, even though Mack recommends it. Any thoughts?