I just put X6 Y-spoke 20" wheels on my X5 4.6is 2003 (E53).
There are a few VITAL issues.
The rear X6 wheels must be bored out to fit on the X5 hub.
So it fits, but why oh why does my steering wheel now vibrate so much over 80km's?
After replacing bushes, changing tyres, re-balancing wheels and going on 2 long trips with a shaking wheel, wishing i owned a different car, a tyre specialist told me that the car was shaking because the hub sizes do not exactly match, so eventhough the wheel bolts on, it is not sitting exactly centered on the hub.
He said you have to put hub spacers on ALL 4 wheels, to fill the gap. He put them on as he has for other X5 owners putting on X6 wheels and voila, $20 later my ride is finally perfect, the way it used to be!
Note: the X6 wheels are heavier than the original E53 4.6is 20" wheels so there is a little clicking noise from the suspension which i am looking into.
But i am so glad my steering shake has been resolved.