This is a much debated topic, unfortunately no cheap solution exists. You need to use the correct ZF fluid. I managed to find it as cheap as $38/bottle - still a steep price. Someone suggested checking wiht a BMW dealer, some 540 and 7 series have the same transmission.

There is a Castrol Import Multi Vehicle ATF, which may work, but is not 100% replacement. Even Castrol says you can experience reduced fuel economy as the new fluid has different viscosity. This to me sounds like a red flag.

Incorrect ATF fluid can have very unfortunate effects on yout transmission and would defenitely void your warranty.

Your 07 XJ should still be in warranty, why do you need to change the transmission fluid? The unit is "sealed for life" - which means it's probably good to at least 60k miles.

Good luck.