Lifeguard fluid 6 is what ZF the maker of the trans specified for the trans. They caution against using any other type of fluids which can lead to failure of the trans.

What about febi fluid?

TRANSMISSION PROCEDURE
 

Hi my friend.
first of all sorry for my english, now i like to write something about transmission change fluid(because i have the same car): first you must open the plug 8mm bellow and dump the old fluid.After dump you put back the plug and you open the fill up plug (8mm i think )from the left side of the transmission(is close to where is connect the electrical loop to the transmission) and start to fill up your fluid.When is start to over flow the transmission you stop and put back the plug.Start the car and go back open the fill up plug and continue to fill up until to over flow again.at this point your are done(you must use between 6.8 and 7.5 lit). and for your info one year ago i use for 800km fluid (no bmw) from gas station normal atf (because i lost the fluid from my transmission) and was with out any problem.i believe is missing fluid from your transmission must do the operation from the beginning.
i hope to help you.best regards.

Thanks all for comments.

My particular model of X5 is actually a bottom fill, never seen this before but apparently its common.

Topped up today as per instructions and it took a healthy drink of ATF, all seems well at present, certainly feels smoother and more responsive but having reset the adaptation as well I'm not too keen on taking it out for a thrashing just yet.

Cheers again!

Sean.