Gearbox oil

Exactly like PAV says. Remember to buy a new copper crush washer. You can use a 1M length of garden hose , stick one end in a cup of boiling water to soften it for a few minutes if the funnel is slightly too big to fit in the end of the hose.
 
Yeah pretty easy just make sure you have plenty of rags/cardboard down to catch any oil when filling back up as it can get messy.