I am working on a 06 5.9 cummins. Its blowing oil out the vent at running rpm. ie like down the highway. no power loss or fuel mileage loss. rough idle when its cold. but when warms up it runs like it was brand new but the oil. stock truck. 120,000 miles

How's the oil level???? Are you "making oil".............leaking injector????

I believe by 06, the vacuum pump was off engine electric operation... (only earlier, vacuum pump seal failure could pressurize the crankcase).Blowing oil out... Usual causes of crankcase pressure... blowby, failed piston... head gasket... How's the oil look? Any water contamination? For that matter how's the coolant, color, condition, bubbles?

the 04.5 to 07 5.9 is famous for melting pistons. the best anyone can figure is due to the non-reentry piston design. I'd almost put money one of your pistons has deformed causing excessive blowby.