The drain plug is a recessed 17mm (I think) allen key plug at the bottom rear of the fearbox.
The fill plug is the same thing on the front of the gearbox an inch or two above the bottom of the box.
On level ground drain the oil whilst it is warm / hot, watch your hands.
Replace drain plug, remove filler plug and squeeze in about 2 litres of oil. It is full when it starts to run back out of the filler plug when the car is level.
Oil required is 75w90 GL4. Buy the stuff from the main stealers and you'll get the right stuff and it'll be half decent.
Make sure you can get the filler plug out before you drain the oil otherwise you'll be up sh*t creek.
The old stuff smells like nuclear cats p*ss.
It is handy if you can get the oil bottles with the plastic filler tubes so that you can squeeze the oil out of the bottle into the gearbox, no way you can pour it in.
Paul