Do you know what kind of gearbox your car has? If you are unsure, buy the AutoDNA report, it will contain the gearbox ID. My car has the Aisin TF-80SC transmission, so anything I write below is only applicable for that.
The best fluid for it is Aisin Premium ATF6 (note. not the ATF6+) or just buy directly from Volvo (much more expensive though)
Last time I did this procedure I bought 6 liters of that oil, which is enough for what you have planned. Of course you can buy more if you wish.
Before you get started, locate the fill and drain plugs and buy the correct torx heads for them. If you try to use smaller one, it will strip the bolt. For the required O rings, you can buy those from Volvo directly.
This video details the procedure:
https://www.youtube.com/watch?v=EAb9p8Wph-M
I also attached the information Vida has to offer for this.