Eventually most of the oil will return to the sump. It just takes a long
time.

It is not common to have both inlet and outlet on the bottom of the cooler.
At low flow rates foam will collect in the top cross tubes. Then at high
flow rates the foam will be moved along the system to a less
helpfull location. In and out at the top of the cooler is more common. Or if
the cooler is vertical, in at the bottom and out at the top.

Lynn E. Hanover