[JNoSol]

It's fine. X5s have slush box transmissions, nothing really wears during a downshift/engine brake. I've been doing that to many of my cars. One catch though, engine braking with these newer engines with low pressure piston rings allow oil to pass-through (oil burn). It's not harmful as the oil will just burn off, but if you have to top oil more often then that's the reason.

Engine braking technique has been used by experience drivers for years. From tracking to going down steep hills, this technique reduces brake heat and excessive brake wear or risk of brake fade during these events.