I have the same issue and this explanation expands a little on exactly what Wingnut said.
This is from the Simrek drive shower site:
The unexplained need to keep topping off your drive oil reservoir bottle, is caused when the oil is overheated to a point were it is expanding and causing more pressure than the drive seals can handle. The factory pressure tests their drives by pumping air into them until they reach 15 lbs. of pressure. If the seals hold the pressure for a certain length of time, then the drive is considered good. The problem arises if the drive oil expands under heat to a point where it causes more than 15 lbs. of pressure. The drive oil is pushed by the seals until the pressure drops under 15 lbs. or until the drive oil is cooled by submersing it in water. This is the point were the drive is calling on the drive oil reservoir bottle to refill the drive. This problem is commonly referred to as “the mysterious drive oil disappearance”. No oil appears in the bilge, anywhere on the drive or transom, or as an oil slick in the water when the boat is docked. After inspecting the bilge and the outside of the drive for the presents of drive oil, the next thing that a Mercruiser Technician will do is to pressure test the drive, only to find that it passes the 15 lb. leak test. Now he is stumped and can do nothing to help you other than say that it’s up to you to keep the reservoir full at all times.