You have a very rare Discovery - nobody bought them.
I suggest that you get a Haynes manual because there are three places in a Discovery that need topping up with gear oil and the gearbox isn't one of them. I think from memory you need to use ATF in the gearbox rather than gearbox oil - check with the manual. The manual will also show a picture of where the oil goes. The other three places are: the transfer box and front and rear diffs.
Terry, November 2006