Oh hell it is a DDEC engine. I didn't know. The lower line is the turbo oil drain. It's o ringed into the block. If I were you at 21 years old, I'd replace all the ATA hoses on the pressure side of the turbo. They are pretty cheap and at 21 years old are on borrowed time. You pop one of those, she will really smoke.
Least it's a pre EGR motor. DDEC engines with EGR are prone to carboning up
in the intake tract.
Most people on here would never attempt what you are doing, they's scratch their butts and take it to a diesel repair shop and pay 100 bucks an hour for the repair. I'm like you, I prefer doing it myself and I'm cheap.
I thought you had a kitty cat....lol