I'd say keep it simple: drain the reservoir with one of them giant syringes, replace with new fluid, and hand it back to your dad. Do this a handful of times with some driving in-between so the new fluid circulates. Did you say you dad's was black? OMG it should be green! I suppose you could also detach the reservoir and give it a good clean the first time you replace the fluid. For reference: I too noticed my dad's 2011 S60 D3 steering was heavy compared to my 2011 V70 D3, and after doing what I've just described his steering feels fine
Good luck
