I don't know about the oil pan leaks, but the valve cover and the front timing chain cover will leak if:

-The sealing surfaces aren't scrupulously cleaned before installation.
-Using the wrong (ie not the black MB or Loctite equivalent) or expired sealant.
-Using sealant in the wrong places.
-If your valve cover is warped, as can happen with prying to remove it or not torquing the bolts down evenly to 8-10 Nm.

Oil pressure isn't the problem here, I don't think any of these gasket surfaces are exposed to the pressurized oil system - they just get splash. I also don't think using synthetic is a big part of your problem, but that is just me.