The official Volvo advisement for this particular vehicle from the owner's manual for the power steering is:

Fluid: ATF
Recommended Change: no change is warranted (no other advisements)

My recommendation based on experience is: grab a quart of Amsoil Universal Synthetic ATF (works in every transmission that uses "transmission fluid") and suction out of the resevoir as much of the old fluid as you can. Then refill with the Amsoil synthetic.

Immediate results will be noted in the smoothness and any residual noise that you may have heard. The improvement will continue the more you drive the car. If you wish, suction out again (maybe because you initial suctioning was real dirty)and refill with what you have left. On mine I use about half the quart for the first refill. This stuff works really great. My advisement is based on my personal experience with this particular product. I have no affiliation with the manufacturer. I can only attest to the results that I have had with my cars.

P.S. I have used this for transmissions flushes and it really improves the whole operation. You can fill in the positive comments. Think of smoothness, ease of upshifting, downshifting, and a remarkable drop in temperature which is very noticeable in 90+ heat on a jammed highway.