I expect that this was due to stuck oil rings. I would suggest a few more very frequent oil changes while the engine is HOT! Get the engine to operating temperature which takes about 15 to 20 miles of driving, even in the summer heat. Pull the plug(or having Jiffy Lube pull the plug, whichever you do) and let it drain as long a possible.
If the engine was gunked up enough inside to stick the rings, there is still alot of sludge that needs to be removed. The above procedure is probably the safest way to do this. You will be doing something very healthy for your engine.
Change oil hot, and change oil often,
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in