AntiFreeze
The oily deposits will have come from the antifreeze in the cooling water - if coolant had been getting into the cylinder.
Check the old head gasket - if you still have it - for signs of "blow-by" and leakage.
I am pretty sure that any Opel head rebuilder would have checked the casting for cracks before expending any effort on it so carefully check the cylinder block for cracks or leaks too.
Carefully check the head/block faces for flatness as well.
Use a top quality Felpro "Printo-seal" head gasket and make sure that none of the head bolts either bottom out in the threaed holes (if your head or block have been surfaced) or have "dozy" threads ( ie stripped threads )
HTH
The oily deposits will have come from the antifreeze in the cooling water - if coolant had been getting into the cylinder.
Check the old head gasket - if you still have it - for signs of "blow-by" and leakage.
I am pretty sure that any Opel head rebuilder would have checked the casting for cracks before expending any effort on it so carefully check the cylinder block for cracks or leaks too.
Carefully check the head/block faces for flatness as well.
Use a top quality Felpro "Printo-seal" head gasket and make sure that none of the head bolts either bottom out in the threaed holes (if your head or block have been surfaced) or have "dozy" threads ( ie stripped threads )
HTH