[Chooch84]

Rear Diff oil spec is API-GL-5 (MIL-L-2105 B), as mentioned in the green book. Your axle should be the 1031, as it's after 91, then you should need 1.75 litres of oil as according to the TP-31685-1 green book. 1031 pre 91 is 1.6 litres of oil.

Gearbox oil (M90) capacity is 1.75 litres, Volvo part number 1161423-7 as taken from Volvo Green Book TP-4304031 which is all about the M90 and written in 1994. Since then the part number for the oil provided has been superseded multiple times, it's the same oil that is used in the M56 gearbox. The original gearbox oil spec is "Volvo 97308".

For my gearbox, I've swapped over to a Castrol oil with Ford spec WSSM2C200-D2, as that has a very similar spec to what is used in the M56. I previously experimented using Redline MTL oils and found them to be "crunchy" in the winter.

As it has been stated, there are other threads that go into depth about oils for the gearbox and diff, and the multiple part numbers associated with them. If you're looking to do the job yourself a bit of research looking into the oil specs will take you a long way if you don't want to go to your local Volvo dealership. Either way, you will not get hold of the original oils used from a Volvo dealership.