PROCEDURE
Fluid Level Checking Procedure for all Transmissions without a Dipstick:
1. The transmission temperature must be between 30°C and 50°C before checking can begin. Use the
DIS or the MODIC to determine the transmission temperature.
2. The vehicle must be level and without load. With the engine running, switch on the air
conditioning. This will increase the idle speed and ensure that all oil passages in the transmission
are filled with oil.
3. Step on the brake firmly, apply parking brake fully and move the selector lever through each gear
position, pausing briefly in each gear.
4. With the engine running and the selector lever in Park position, remove the filler plug located on
the transmission (see picture for location). If a small stream of oil runs out, the fluid level is
correct.
5. If no oil runs out when the filler plug is removed, the fluid level is too low. Add oil until it starts
to overflow.
6. With the engine running, reinstall the oil filler plug and tighten to proper torque.
F below for proper tightening torque)

Being low on fluid can manifest into adverse characteristics.

Thank you very much. Did not know the air con had to be on, and that I could fill with the car running. Thanks.

ZF filling procedure
 

Here is a link to the ZF spare parts catalog that has the definitive & more detailed procedure in the front of it.

http://www.mwrench.com/Whitepapers/ZF/5HP24.pdf