For the BMW 528i power assisted steering BMW specify Dexron II/CHF 7.1/11S. This particular oil will not deteriorate rubber seals in the system, nor re-act with metals in the system, it's useful life and performance are assured. Once upon a time automatic transmission fluid was just automatic transmission fluid, its not now. Stay safe, use the correct spec, a litre of Dexron will always be cheaper than a new rack or pump or an accident. If the system is leaking then it needs to be repaired. Leave the repair and you can bet your last dollar it is going to let you down at the worst possible time.