1998 Mercury 9.9/15HP 4-stroke outboards factory service manual, Page 24Get this manual
MAINTENANCE
Positioning Outboard for Storage
1To prevent problems which can be caused by oil entering the cylinders from the sump, store the outboard only in one of the three positions shown below
aUprightTiller Side DownBack Side Down If outboard is stored tilted up in freezing temperature, trapped cooling water or rain water that may have entered the propeller exhaust outlet in the gear case could freeze and cause damage to the outboard