adara wrote:

From the L322 07-on Service Sheet:
Replace automatic transmission fluid and filter every 10 years/240,000 km.

Land Rover don't make the transmission - so I would be inclined to go for what the manufacturer of the box recommends. 

From WIKI-P (for whats that worth) :

Transmission(s) 8-speed automatic[6] (since 2010)
6-speed automatic (since 2006)
5-speed automatic (2003–2005)

I e-mailed ZF and got the following reply:

5-, 6- and 8-speed as well as 4HP20 automatic transmissions:
ZF 5-, 6- and 8-speed as well as the ZF 4HP20 automatic transmissions are filled maintenance-free with specially developed partially synthetic ATF oils. Maintenance-free fills are intended for normal operating conditions. Especially driving at very high operating temperatures can result in accelerated aging or increased wear of ATF oils. It is recommended, in the event of severe operating conditions, such as:

- frequent highway driving in top speed range,
- offensive, sporty driving style,
- frequent trailer operation,

oil purification (oil change) on automatic transmissions is recommended between 50,000 miles and 75,000 miles, or 8 years, depending on the load.

In each case, only released ATF oil may be used for oil changes. And oil changes must be performed in accordance with the relevant specifications.

SO they treat the 5, 6 and 8 speed boxes exactly the same.