Another point to emphasize is when you're repairing the crankcase breather covers mounted on top of the valve covers, these are fastened by aluminum screws. These screws are a 1x use only. They are very susceptible to stretching when torqued too high and the proper torque setting is very low (4 lb-ft = 5-6 N-m). Mercedes-Benz states that these screws are to be disposed of if removed. So if you used these screws over again, and over-tightened them, your problem may be the screws.

Another point I found when I repaired my crankcase breather covers. I checked the tightness of the star point screws holding down the valve covers. In all cases, these screws were loose on both valve covers. Unlike pizzaguy, I did not remove the valve covers during the crankcase breather repairs. I just re-tightened these screws to the proper torque (10 N-m = 7.5 lb-ft). The valve covers on my XFire have never been removed or leaked. And I check these probably 1x/yr to make sure they are still tight.