Don't put the Marvel Mystery Oil in the Engine Oil. I am not in favor of diluting your Engine Oil with anything.
You can remove Glow Plugs or Injectors and dump it into the Cylinder. The catch is you need to leave it there for a Week or more. Somewhere inbetween rotate the Engine a little and add some more.
This only helps if you have sticking Piston Rings.
The Marvel Mystery Oil soak worked on my Volvo that had hazy gray smoke all of the time but did nothing for my Mercedes.
Some say you can achieve the same results by switching to Synthetic Oil and driving 200-300 miles or more that it takes to work.
Some people have removed the top of the Breather in the Air Filter and sealed the edges with Silicone Sealant and had that work. Sometimes the Drain Tube or Check Valve is clogged.
This was my solution:
Washable Air Filter and Intake Mod , With Larger Breather
Washable Air Filter and Intake Mod , With Larger Breather - PeachParts Mercedes ShopForum
Some people have bought or made a larger Crankcase Breather. Some of them just go down to the Ground insted of into the Air Intake.
Bad Valve Stem Seals and worn Vaves/Guides can cause Blow-by at idle.
I replaced my Valve Stem Seals; cheap but a little work. It did not help me to the extent I could tell the difference.
My own thouth is that having the correct Valve timing and Fuel Injection Pump Timing keeps the combustion happening when it is supposed to and the Piston in the proper Position whe ignition starts could also reduce Blow-by.
If you have been using an alternative type Veggie Fuel that has been noted for coking up Piston Rings.