Greasing the differential
First release the filler plug (in order to be sure that once drained the differential can be refilled..). To release the filer plug you need either the Britool adapter that comes with the car or an half inch Allen wrench (12.7mm) . But other users have succesfully using a 14mm allen bit (to fit a 1/2 square socket) filed down to 13,6mm accross the flats.
Source: The Rolls-Royce Forum - Australia

Why oil and not grease into the trunnion joints ?
Because due to centrifugal force grease would settle in the wrong part of the rubber boot causing imbalance (Patrick Lockyer)
Source: The Rolls-Royce Forum - Australia

Greasing the trunnion joints
Has to be checked every year.
The fill / drain plug is located on the oval shaped housing.
The trick is to position the drain plug at the clock position for either 4 or 8 o'clock to fill and let excess dribble out.
The filling is uneasy (better to use a bottle attached to a spout) or a syringue.
150ml of Castrol SAE90 EP oil for each trunnion.
Source: The Rolls-Royce Forum - Australia