Non-Star rated tyres on X-Drives:
My advice on using non-star rated tyres on an X3 is be very careful. I used to own a 58 plate X3 from new, which had Dunlop star rated tyres on, got about 25k miles out of them and changed them to a set of Michelin Pilots (not * rated). Noting that the tyre sizes front to back on that vintage of X3 were different as standard.
Had loads of rumbling & hesitation from what I thought was the transfer box, so took it back to the BMW dealership, who changed the transfer box and rear diff oil, made no difference.
To cut a long storey short it went back lots of times over a fair amount of miles whilst changing quite a few bits to fix the problem. The rumbling noise/hesitation was still there. I eventually replaced the Michelin's to Bridgestone's & it made the problem even worse.
At that time, I'd had enough and took it back to the same dealership yet again.
Ended up with their master tech. looking at it. Straight away he swapped my wheels for a an identical set from another X3 they had in the workshop (with star rated tyres on) and the problem went away, it drove like it did when I first bought it, unbelievable difference!
He then measured the rolling circumference of the Bridgestone's and they varied hugely from front to back & wheel to wheel.
In summary, he advised me that the differences in rolling circumference were out of tolerance for the spec. of the x-drive electronics/transfer box and told me I needed to put a set of star rated tyres back on. Reluctantly I did and the problem went away for good. He measured the rolling circumference of * rated tyres and I could see they were much tighter in tolerance. I ended up doing another 40k+ miles and had no issue before I sold it at about 100k miles.
There is a link here: X Drive Transmission Wind Up | BMWLand
Runflats:
Following on from the above, I had a 4series (non x-drive) on Bridgestone run flats, hated them, handling was awful. Just before I sold it I put non-run flats on and it transformed the cars handling.