Hello BlizzardMan and welcome to the AMPS forum,
You can use any gear oil but I prefer a marine gear oil like Lucas because it will hold up better in the event that some water makes it's way into the case.
( I found some at my local auto parts store)
Keep in mind that there are two places on that out drive which require lubrication, the top half and the lower half are not connected. you should fill the lower half with oil up to the fill hole when the outdrive sits level. the purpose of the second fill hole is there to allow you to flush out the old oil with WD40 and is only found on the later outdrives.
It is important to not "completely fill" the reservoir because as the gears begin turning at high RPM's it will make the oil expand.
I use a little teflon tape wrapped 3 times around the treads of the grub screws to keep the water out.
The gears will hold up fine as long as you don't hit bottom or run through weeds.
Have fun and enjoy your new boat!