After holding to the silly position against changing the fluid for some time, Volvo finally came out with several service bulletins advising that under "severe" conditions, including certain symptoms, fluid colors, etc, it was advisable to flush the fluid. Ask to see those bulletins. You are carrying around all kinds of contamination and abrasives in that old fluid. Lots of instructions around on how to DIY flush including Bay 13. I swear by the flush.
The transmission computer will adapt to the characteristics of the new fluid after some driving time. I use only synthetic fluid- Mobil 1 ATF or Amsoil ATF.