My 94 disco (226,000 miles) has leaked some gear oil due to a "not so bright previous owner"

and i was wondering what is the best way to add more to the gear box behind the tranny? Is there a specific type of lube I should use? I was thinking of using a "restoring" type, I used before (cant think of the name) but it was quite thick, almost sticky, trying to keep any possible leaks from happening.
Any ideas?