Hi There - I'm the one who had to get his 560sel valve cover milled. Over a year now and still perfect. The reason they warp is NOT poor quality - it's over torquing the bolts that hold it down. Any good machine shop will be able to mill the bottom edge for you. Mine had to have 7 thou milled off to level it. New crush washers and a torque wrench set to 4 foot lbs - no more problem. Yes I was quoted $600 for a new cover but the Mercedes Dealership told me that they could send it out to get milled for about $150. I went to a shop that mills valve covers for racing engines - apparently they are notorious for warpage from the extreme conditions they are subjected to. The shop charged me $70 CDN - well worth it. If the 190's are doing the same thing - buy up all the warped covers you can - mill them and re-sell them for 50% of new - every one wins!! Just - remember - DO NOT OVER-TORQUE THE BOLTS!!!!
Signed - learned the hard way.