98 E320 valve cover oil leak
When I arrived home last night, I noticed an oil burn smell from the engine area. I removed the plastic cover from the top of the engine and noticed some oil gunk around the left valve cover and all over the electronic stuff (guess they are coils) attached on the top of the valve cover. I also noticed that small amount of oil is seeping down on the manifold causing the smell. I tried tightening down the 3-4 valve cover bolts which were visible and accessible without removing any engine parts.
Now I have a few questions:
1- Is this a common problem with 98 E320s?
2- Is there anything else I can do to stop the leak before taking it to the dealer?
3- Can such oil leak on top of left valve cover and coils be from anything other than valve cover gasket?
4- If tightening the bolts helps, do I need to remove the air filter and other engine parts for accessing the other bolts less visible?
5- If I need to take it in to the dealer, how much should I be expecting for such repair cost?
I thank you in advance for your help.