Ive experienced this problem. I believe the problem is from oil leaking into the head bolt from the top and running down the bolt, out the side of the head. If you look at a head gasket you can see how this possible, this area is off to the side of the head gasket off the metal plate which protects the cylinders and cooling passages. And the likely cause since oil drains only at the rear.

in theory this is caused by head bolts with insufficient torque allowing oil to enter.

Ive noticed many times that there will be gunk stuck to the head bolts on disassembly. From oil leaking in. So its common. Only once have I seen it leak out the side of the Engine. Engine works fine otherwise.