most Benz valve/cam cover bolts need to be torqued to ensure even sealing of the gasket surfaces. To do otherwise or to overtighten will invariably cause a leak.

On my M117 engine the torque value is very low, specified to 3 to 4 Nm. Check your cover torque spec and use the torque wrench to achieve it. You may have to get a 1/4" drive beam-type torque wrench to get it in the center of the range where accuracy is best.