Get a Lower Unit Lube 80W - 90 from any marine supply store.
A tube should be enough, you will not need very much, no need to purchase a quart unless you you want more to change it again next year. (hint - hint)

Here is a site that show an outboard lower unit lube being changed:
http://www.nitroowners.com/projects/lowerunit/lowerunitproject.htm

Not your motor I know, but almost all outboard lower units use the same fill and vent set up.

Good Luck and Boat Safe ! !