My local Volvo dealer uses Shell Helix Ultra (the 5W 40 grade) for petrol engines. I've never used it but it is a name brand fully synthetic oil so I assume that it is very good. They highly recommend flushing the engine at every oil change but this is at the owner's discretion i.e. Volvo do not include it in their standard service schedule.
Personally I use Mobil 1 0W 40 grade and always flush the engine before replacing the filter & putting the new oil in.
If just buying gearbox oil for topping-up, try to use the same oil as is already in there. If in doubt, buy a small bottle from Volvo as that is most likely the stuff that's already in it.
If you are planning to replace the gearbox oil completely then use a good name brand fully synthetic gearbox oil that meets or exceeds the specification detailed under "Oil Specification" in your owner's handbook. You don't mention whether the transmission is manual or automatic - they use different oils so do ensure that you use the right type.