We do a fair few transmission fluid changes ( I'm the service manager for A1BN www.a1bnspecialist.co.uk
) and to be fair the biggest hurdle we kind of come across us telling customers that the fluid is around £19.20 a litre (we buy 20 litre drums from our local BMW dealership) which kind of blows the sealed for life statement to bits otherwise how/why would I be able to obtain the fluid so quickly? For the average car you'll drain it(though not 100% and put around 6 odd litres back in) which means the fluid alone is over £100 with ease add the sump and labour and you're on the £400 mark in some places as stated. Time wise it's a good 1.5 hours to do.
Then you go on to the sump made of plastic there around £165 odd plus the VAT and the filters are integral so you're kind of stuffed re doing a proper job unless you buy the sump. We've seen fair few 3's & 5's with sweating sumps(they seem to go on the offside towards the rear) the annoying thing is its all to common hence getting a sump if ordered before 10am for me means they'll get it for me the same afternoon so again blows the myth wide open re sealed for life IMO.
When we offer this service we also offer the option of updating software On the gearbox side of things to the latest version explaining that in the world we live in various things on modern cars are constantly evolving and it can be a worthwhile exercise to see if an software update is available as it can help elevate some behavioural issues plus various bugs and fixes may also be available within the update that may through time of been discovered.
I'm going to do my 335i soon(purely as I think it's time on age/mileage) but its a worthwhile investment IMO having already of done the software update this can only help.
There is of course nothing stopping you changing the fluid but the full effect may not be gained unless the filters changed but if you see the sumps we see when they start to sweat oil it's almost game over and they'll need changing at some time soon.
Going back to the sump quickly BMW use a plastic sump so I understand there are a few manufacturers using the same gearbox hat have a proper metal sump(a good customer of mine told me this) but I dare say that there maybe some differences re something or other that'll stop one of these being used.... A case for more research I guess.
One thing finally that's really surprised me is how finicky this auto box is in its various applications compared with the more basic version that was on my old E46 330d it's adapts to your driving style with more of a noticeable difference than the older verity of box,which I'm now coming to terms with. I can now accept the engineered amount of slip on my car in the lower gears which although frustrating can as I'm getting used to be be "driven around"
Re the diff TBH haven't come across a car that's had a fluid change so it'll be difficult for me to say but I dare say it'll take a good 1.5 to 2 litres of fluid, it would be cheaper of course than he eye watering Auto fluid price on X5's for instance the recommended fluids around £13 from memory.