1998 Mercury 9.9/15HP 4-stroke outboards factory service manual, Page 299Get this manual
BIG FOOT
Draining and Inspecting Gear Lubricant
WARNING
If gear housing is installed on outboard, disconnect (and isolate) spark plug leads from spark plugs before working near the propeller1With gear housing in normal running position, place clean pan under housing and
remove vent plug and filldrain plug (with gaskets)
a-Vent PlugWasher b-Drain PlugWasher