Hi I just changed my fluid Lifeguard 8 around 100k miles. Lots of metal in the pan on the magnets and really glad I replaced all that crap.
If I was only going to do it once at 100k I would use the Lifeguard fluid. If I was intending to go on regular interval changes like non lifetime fluid I would deffinately consider the ams fluid and others but when the clutch packs get old they need a certain amount of friction material in the fluid. There are even additives if the trans starts to slip you can add and prolong the life of the transmission. It is more likely that this will be needed if you are changing the fluid more often later on in the high mileages vehicles.
Other than that, I don't see a reason personally not to use the ZF8 specifically designed for the transmission. Viscosity values only tell you one part of the story but even the manufacturer ZF themselves recommend changing it although at high intervals, however BMW says not to. This is likely based more on marketing than on engineering reasons.
Last edited by wvadam; 03-15-2019 at 10:22 PM.