To service the gearbox you should take it fully apart and clean out all of the old grease. This usually requires soaking the parts in white spirit, especially if some of the old grease has become toffee-like. Most of the gears have shim washers, take extra care not to lose them when cleaning out/disposing of the old grease. The gearbox should be re-packed with 133 grammes EP2 lithium grease. Clean the sealing faces and reseal with silicone sealant (the small tubes available from some pound stores are ideal for this).
Nowadays, food safe grease is usually used around the teeth of the large ring gear for the planet hub, although the original install was the same as for inside the gearbox. Do not put food safe grease inside the gearbox; it doesn’t provide the same degree of lubrication to the gears at high speed and will result in damage.