Oil leak question for my 2004 5.9L Cummins 2500
 

So I have a 5.9L 2004 Cummins and on occasions, it'll leak oil. I'm pretty sure it only leaks after you have run the truck pretty hard for a long period of time. Like when I pull a car across state or something. Then after it clears up, it seems to not leak when just driving it around town from day to day. Are there any common issues that resemble this? Thanks--Andy