I am assuming that the chronic leak you are referring to is the crankcase ventilation system leaking via the oil seperator in the air cleaner. This is definitely difficult to permanently fix. I'm struggling right now with my daughters 300D.
But what I really want to comment about is someone making a blanket statement saying that all diesels leak. I think that is a totally absurd thing for your mechanic to say. I wonder if he's trying to dodge something. Yes, there is a problem with the vent system on the five cylinders, but I've operated diesel tractors, Dodge Cummins and Mercedes Benz diesels and NEVER had ANY leaks except the vent on the 300D and the rear main seal on my 240D after 380,000 miles, otherwise NOT A DROP. I would be suspect of an automotive professional that made a statement that "all diesels leak".
BTW, there is a post of about a month ago regarding leaking drip tube. In that post someone mentioned an udpate to the oil baffle inside the valve cover on a 5 cylinder. I have yet to do this, and I will post the results when I do.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in