It's quite normal for there to be a bit of oil leak from the valve when the drive is removed... The other thing to be aware of is that there is a large air space in the top of the drive after the oil is filled up. When the boat is put back into service, that air eventually makes its way out of the housing and up the line to the reservoir bottle, and the oil in the bottle makes its way down and into the drive to replace that air.... Normal to see quite a bit of oil 'disappear' after a service. One of my little tricks to stopping that happening is to fill the drive normally (before I put it back on the boat), then lay it on its starboard side and remove the vent plug. I then top up the oil as much as I can. This reduces the air that will be displaced, and stops people calling me with "my drive's lost oil"....
Chris.........