Smoke coming out of where you put engine oil

My guess would be that too. The mack E-7 etech has a filter that's often neglected or forgot about and they'll plug up on you

 

How many miles are on the engine?

 

That's a lot for an engine lol my dad's C15 has over 700 on it too and it has blow-by so bad you have to leave the engine fan on to even see. I hate to say it but yours may need rebuilt soon

 

Mine got blow by too . 440k miles

 

Where or is there a pcv valve.

 

That's blow by you're seeing combustion gasses.

One of the things you can do to see if you want to buy a truck with a diesel is to take off the oil fill cap while it's running and put paper towel on top of the hole. If the towel blows off the opening the engine is worn and needs a rebuild so either don't buy or take about 10 grand off the price they want for the truck. (small truck, like F-350 size).