FSM states to use Oil Dipstick 8863 which will be a Miller Tools number. Googling that you need a 1200mm Merc 722.6 dipstick.
STANDARD PROCEDURE - CHECK OIL LEVEL
(1) Verify that the vehicle is parked on a level surface.
(2) Remove locking pin (1) (Fig. 111). Remove the plate of the locking pin with a suitable tool and press out the pin remaining in the cap downwards.
(3) Remove cap (2).
WARNING: Risk of accident from vehicle starting off by itself when engine running. Risk of injury from contusions and burns if you insert your hands into the engine when it is started or when it is running. Secure vehicle to prevent it from moving off by itself. Wear properly fastened and close-fitting work clothes. Do not touch hot or rotating parts.
(4) Actuate the service brake. Start engine and let it run at idle speed in selector lever position 9P9.
(5) Shift through the transmission modes several times with the vehicle stationary and the engine idling
(6) Warm up the transmission, wait at least 2 minutes and check the oil level with the engine running. Push the Oil Dipstick 8863 in up to the stop and pull out again, read off oil level, repeat if necessary.
(7) Check transmission oil temperature.
( 8 ) The transmission Oil Dipstick 8863 has indicator marks every 10mm. Determine the height of the oil level on the dipstick and using the height, the transmission temperature, and the Transmission Fluid Graph (Fig. 112), determine if the transmission oil level is correct.
(9) Add additional oil if necessary. Use Funnel 8908 to add oil.
(10) If the oil level is above the correct height, use Pump 8910 to remove the excess oil
(11) Re-check oil level as necessary.
(12) Once the oil level is correct, install a new dipstick tube cap (2) (Fig. 113) and lock pin (1).