Are you actually getting drive lube in the bilge, or is it simply disappearing in the overflow reservoir? Are you seeing lube on the outside of the reservoir bottle?
One thing to keep in mind, the outdrives are filled from the bottom of the drive, not the reservoir on the top. If the drive is low, it won't drain easily from the reservoir to the drive. If some idiot tried to fill the resevior, it would appear full, but would keep dropping down as more fluid was added. The other thought is the resevior overlows and purges out the top if the drive is overfilled from the bottom. The technic to fill a drive is to pump up from the bottom until you begin to see fluid in the bottom of the reservoir. Then stop. It typically continues filling to about 1/2 way. Then you can top of the rest in the reservoir. When you pump from the drive up until the reservoir is full, then once you start running, it continues filling the reservoir and it overflows.
You might be dealing with one of these two conditions.
I think we need slightly more information.