What can we use to top off power steering fluid? Donax TX not available in quarts.

Hi gang. Search didn't help, so turning to the collective wisdom of this forum . Need to top off power steering fluid, which is slightly above MIN. I understand Donax TX is a glorified Dexron ATF fluid (same one specified for F1 shifting mechanism), but want to make sure the suggested alternative to just top off the reservoir (couple of ounces?) is fully compatible. Hope I don't have to flush the entire system as it was just serviced before buying the car. And with closest Ferrari dealer 400+ miles away, topping off there is not an option. Thank you folks.

 

I need to order Pentosin CHF-11S for my E92 M3 which is also close to the 'min' mark since new, and it'd be perfect to use it on the 430 too, but is it fully compatible with the Donax TX? Is the 11S also a ATF variant? To be honest, don't know if the PS fluid was replaced; need to check again the maintenance history, but doubt it with only 9K miles. I usually suck the PS reservoir dry and fill it with new fluid every 30K miles or so, but with such long lines on the 430, don't know if that'd be effective. Any thoughs on that? Maybe doing it every 10K miles can avoid a flush at 30K, which is obviously more difficult. Thanks man.