Truth be told in this situation, you never will get the full measure of oil out of these as if they were dry. There are pockets, cavities and hydraulic piston assemblies that will all retain a small amount of oil in them.
Don't be alarmed because this is natural due to design.
Top the units off and purge the air with the wheel off the ground and cycle the motors forward and reverse several times. I usually let a unit it for about an hour and then repeat. This makes sure any trapped air in the oil has a chance to escape.
