Ok, Probably everyone knows this.
When I first took off my 300TD cover I marveled at the rubber gasket thinking that unlike the cheap cork ones..I can use this again! When I adjusted my recently acquired 300D's valves and noticed it's gasket was brittle and cracking with the slightest bend
I pulled out a used one I had....still felt soft and looked good.
But...after installing it, the thing leaked like a sieve!
I applied a .0025" feeler gauge and managed to easily slip it between the seal and the head at engine rear. Mercedes limits the amount of squish with bosses on the valve cover, so depending on the flatness of your cover and head surface there is just so much available compression of the rubber seal.
.....so just pay the man the $10 for a new one. At least there is no scraping involved

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And while you have the cover off, it's a great time to adjust the valves!

On occasion the valve cover itself will warp. Best way to fix that is to set it on a big piece of glass. Mark the spots where it makes contact and then get a flat sander and take off those high spots. Do it carefully and you can quickly restore the valve cover to a nice flat seal against the gasket. No more leaks.