I have had a timing cover leak on my 300E for about 80K miles. It doesn't even leave any drops on the garage floor. This leak is not uncommon with these engines and has caused no trouble whatsoever. I wash the engine occasionally anyway.
As long as the leak is slight enough to not drip underneath the car, forget about it and enjoy.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in