That means that the oil is coming from an un-pressurized source, like a return galley. When you block the vent the air pressure increases in the rocker cover and helps push out the oil.
Sounds like there is a fair amount of blow by.
Did you check that the head was flat and there was no damage to block or head, in this case, around the oil return holes and push rod holes?
Most likely oil returning from the rockers is pooling on a mis-match (common) of gasket or castings at the head/block joint and the pressure is helping it find it's way out through a bad gasket or poor sealing surface.
