Blow by
 

My 74 240d smokes a decent bit at idle when cold. Not so much when it get up to temp. I did the oil filler cap test and there was a decent amount of smoke. The car doesnt burn that much. Or i havent noticed. Starts and drives like normal. What are some ways i can slow it down from getting any worse?

Keep the oil fresh and don't overheat it! Don't forget to change the injection pump (IP) oil when you do the engine oil. Do some fine tuning - check cam and IP timing, and the condition of the injectors. Make sure the valves are adjusted properly. All basic maintenance items taken care of will extend life for a long time.

That said, it's pretty amazing how far they can go with almost zero maintenance.

If the Cylinders, Pistons and Piston Rings are worn there on remedy but a Engine Ovehaul.

If Piston Rings are sticky people say switching to full Synthetic Oil and after driving 300 miles or so has been know to free them up.

You can also dump something into the Cylinders and let it soak for several weeks. I used Marvel Mystrey Oil on My Volvo Diesel that had gray smoke all of the time and it fixed that. When I did the same to tne Mercedes it did nothing.
Soaking or the Synthetic Oil won't fix sticking Piston Rings if there is none sticking.

Anoter possible is the Valve Stem Seals and or Valve Stem to Guide clearances are letting exhaust gasses past them.
Slightly complicated to change but cheap.

I changed the Valve Stem Seals but there was no noticalbe improvement in the blow by.

Other say as long as blow by causes no issues with the running of the Engine don't worry about it.

In reasonably warm weather run straight 40 weight diesel oil. Helps an engine with existing wear as well.