I just found this in a Jaguar forum, apprently from ZF themselves:
http://www.jaguarforums.com/forum/xj...ce-more-55546/
Dear Distributor:
It has come to our attention there is confusion in the field regarding service intervals and fluid recommendations for ZF 5HP and 6HP applications.
ZF has promoted lifetime service for many 5HP and 6HP factory filled transmissions. Lifetime is not clearly defined and each OEM manufacture has there own philosophy. ZF stands behind lifetime recommendation where listed in the vehicle operator’s manual. ZF engineered and tested our fluids to retain its characteristics through normal lifetime operating conditions.
However, a spirited driving style, special environmental considerations and other individual driving factors may place higher than normal loads on transmission fluids. To assure proper operation it is advisable to perform a drain and fill at 100,000km/62,000 miles or eight years.
Due to the complexity of operation, high output of current engine offerings and the expectation drivers place on their ZF equipped vehicles ZF only endorses the use of ZF Lifeguard Fluids and parts or OEM approved fluids and parts. Fluids and parts not approved for use by ZF or the original equipment manufacturers have not been tested and may not meet the required demands.
Use of these fluids or parts will void factory warranties and warranties placed on aftermarket transmissions remanufactured by ZF. The use of any fluid additives is prohibited as the result will unpredictably change fluid properties potentially resulting in transmission failure.
Some fluids previously recommended were factory fill or available only in the European market. To simplify obtaining the correct recommended fluids ZF now offers Lifguardfluid5, Lifeguardfluid6 and Lifeguardfluid8.
Finally there are some customers that feel the need to perform a transmission oil service as part of their regular maintenance. Changing the fluid more often is entirely their decision. Properly performed oil service will not damage the transmission.
Most problems occur due to improper refill procedures, incorrect fluid and or non OE parts or with transmissions that already have a drivability problem. Use of ZF or OE parts and fluid is strongly recommended for best results,