Remove the lower plug briefly and watch for water to come out or for the oil
to be milky. If this is the case the oil is contaminated and should be
drained. Find the upper vent screw/plug, remove it and completely drain the
lower unit.

If water is found then there is a seal problem, which could be as simple as
a length of fish line wrapped around the prop shaft holding the seal open,
or it could be a damaged or worn seal which needs to be replaced. (There are
several seals in the lower unit).

If there is no water, temporarily replace the lower plug and get your oil
supply ready for use. When ready, quickly remove the lower plug and attach
the fresh oil supply. Next remove the top vent plug and fill the lower unit
until good clean oil flows from the vent hole. When this occurs, replace
the vent plug. Remove the oil source and quickly replace the lower plug.

That's all folks!

Don Dando