just put what it says in your hand book in, i think its sae80. Dont go for anything fancy as gearboxes can be a bit touchy and may not work properly (synchromeshes and stuff) if you dont use the right oil.
Whatever you do dont put oil in the plastic cap thing you are missing thats where the clutch and flywheel are, that caps not mega important so dont worry too much if its missing.
You drop the oil from the big allen key screw on the bottom of the diif and fill it from the same type of screwplug on the passenger side, with the wheel off. If you havent already got one, gsf sell the big allen key required, its not too pricey.