I read the sticky on changing the lower unit oil. My question is regarding filling the unit through the "drain/ fill hole". I assume the motor is sitting upright during this procedure, so I'm wondering why you wouldn't pop the top and bottom plugs out, allowing the oil to drain the bottom hole, and then put the drain plug back in and fill through the vent hole until it's full (similar to my manual transmissions/ transfer cases on my Jeeps)? What am I missing here? Why do you fill through the bottom hole until the level reaches the top hole?