Just to throw another variable into the equation, do you know the history of the car? If the steering rack has ever been changed they may have used ATF in the new rack.

My 03 plate S60 has always had the same red fluid in the steering as is in the auto box, however the other two have CHF in them, and there is no record of it ever having a replacement rack, but over 200,000 miles later the rack is fine.

Stick with what's in it, if you mix them it will ruin all the seals in the rack, not worth the risk, in my opinion anyway!

Unfortunately AG no history with the car. As I've said previously the car was not well looked after by previous owner[s] for example the n/s front spring actually touched the inner wing which a new[second hand] strut/spring assembly cured,so yes the steering rack could have been replaced at some point.Using red ATF for top ups hasn't produced any problems thus far and the pump doesn't make any untoward noises,so I think we'll stick with red for now and if anything does crop up go for a flush and change to CHF.