Re: Lower Unit vent screw location
There's a drain screw in the lower forward "bullet"-shaped section of the gearcase. I'd wager that's the one you found.
The vent plug is higher up, on the same side, located above the anti-ventilation plate (Large fin going around the upper perimeter of the gearcase). It's a metal plug, same style as the drain plug. Note you'll also find a plastic plug near the vent, that's not the one for oil but is for a special flusher that screws into the side, that Merc used to sell.
To drain the lower unit, position your catch-pan below the drain plug and remove the drain (lower) plug.
Then, remove the vent plug. Once fully drained, refill from the lower (drain) hole with "Hi-Vis" maring gear oil until a clear stream of oil flows out the vent plug.
Reinstall the vent plug first. This will prevent the gear oil from rushing madly out of the drain hole when you go to reinstall the drain plug.
Don't forget new gaskets (fiber washers) for the vent and drain plugs, they're cheap!
The gearcase takes around 22-25 oz so if you pick up a quart bottle of Hi-Vis, you'll have plenty. Napa will probably have the gear oil and a pump which you can screw into the quart bottle (and makes it so easy to fill the gearcase!). Your local Wal-Mart may also carry the stuff in their small marine display back in Sporting Goods.
HTH & enjoy the stinky job, one of my favorites (NOT!).............ed
