Tranny fluids are generally interchangeable, although there's alot of superstition that they are not. I've mixed castrol dexron 3 mineral tranny oil with zf lifeguard synthetic tranny oil with no problems now for 3 years. So I suggest that you just top up with whatever was correctly rated for your transmission according to the Bentley manual, or its equivalents as you discover them to be when you google around for the info. Don't be concerned about mixing oils.
Check the troubleshooting tables attached for more info on probable causes and solutions to your tranny's symptoms.