You should use Volvo's gear oil. It is a synthetic and normally
does not need changing. Here is what I see at alldatadiy.com
(worth the $25 subscription for this kind of info):

Volvo Synthetic Transmission Oil P/N 1161423-7, Or Equivalent

Capacity 2.1 liters (2.2 US qt)

On another page it listed the fluid as:

Specified Fluid Volvo synthetic P/N 11 61 423

I've seen it on the shelf at a local independent garage. I bet
it's been on his shelf for years :)

I have found it hard to get fluids shipped by mail from discount
parts dealers so you may just have to get it from a local dealer.