Remove the top vent and bottom fill screws. Fill from the bottom hole till it squirts out the top hole. Uses about a quart, but buy 2. I use a gallon jug with a pump. Good to use new washers on the screws.
A 1968 55 HP is a hydroelectric shift and requires Type C marine gear oil, not the normal 80-90W gear oil.
Available here: http://www.iboats.com/Gear-Lube-Typ...0071401--**********.724307186--view_id.187329