When working with the special sealant, time is of the essence as the sealant sets rapidly. So, be certain that you are actually ready to install the cover before adding the sealant. You must also be careful not to deform the funny little gasket as you install the top chain cover. It is easy to do so and you will end up with a substantial leak. A dry run or two w/o sealant is a good idea. My top gasket leaked almost from new on my '90 300E 2.6 and was re-sealed by the dealer at the 30K mile service. The dealer at that time used red silicone sealant. A BIG mistake. The gasket started leaking again in less that 10K miles. It has been more than a year now since the last re-seal and the area is dry as a bone. When I did mine a little over 1 year ago, the tube of sealant was $12.50, retail, from my dealer. If bought in a full size tube (like household caulk) from a commercial/marine supplier, the price was around $25, at that time.

It is also possible for the valve cover gasket to leak along the sides. At one point, mine did so along the plug (right) side of the engine. During a routine safety/etc. inspection at the dealer, a trained Benz mech. told me I had a head gasket leak. A new valve cover gasket plus a good cleaning and the "head gasket" leak was cured! Like others, I don't like to use sealant on this gasket as it makes replacement troublesome. Have been there before on older cars where that gasket was glued on at the factory. On the M103 engine the job is pretty easy and quick so no big deal if it leaks again. Besides, it gives us a reason to poke around in the engine bay again.

I would definitely go with the Mercedes gaskets for these critical areas as you can then be certain that you have the better quality and the latest versions.