Paccar questions/reviews

[Hetdiesel]

So the owners of the company I work for are looking at buying peterbilts with paccar mx13s in them I was always told and thought they were junk. I read cummins helped with this engine or just the after treatment? Who tunes them? Are they going to be a nightmare for me? They are used and around 400,xxx miles so warranty will be off soon I think.