I think I remember that you have the Volvo TMD22 engine. If so, this applies to you...
You have to be a very careful talking about the ZF transmissions. The ZF25M transmission that came on the Volvo TMD22 is a manual shift transmission, and holds about 0.75 liters of DEXTRAN IV automatic transmission fluid, NOT 90 weight oil. The same exact machine was called an HBW250 before ZF and Hurth joined forces.
Other engines used a ZF25 transmission, which is a hydraulic shifting model with larger horsepower capacity and a completely different design. It causes no end of confusion with model numbers so similar and actual designs so different!
The recommendation for change interval in my manual for the ZF25M is "Once a year." There is no change interval recommended based on hours of operation.
Annapolis, MD, USA