The outboard should be in the down or normal operating position for this
btw.

When outboard engine will be in the vertical position for lower unit drive
lubrication, we will fill gearcase completely with oil, making very
efficient hydraulic engine brake.
My "Clymer Mercury Service/Repair Handbook" states on page 22:
"The gear housing cannot be completely filled otherwise"!
Following illustration #16 shows outboard in maximum tilted position during
filling procedure.
Regards!
Tadeusz