Why is my crankcasae vent valve smoking
there is smoke coming (from the crankcase vent valve) out the tail pipe
You'll want to take a look at the TSB / SI B 11 08 03. It's a well-known problem for this era of BMW cars including X3 and 5 Series.
Here is a copy that I found:
By the time I identified it on my X3, I had already blown a head gasket, but hopefully your X5 will avoid a similar fate.
If your engine is smoking out the exhaust, and sucking oil into engine, which I had happen, it is usually a sign that the two CCV hoses which cross over the back of your engine over the intake tubes if engine cover is off on the 4.4 motor X5’s, squeeze hoses, if they are spongy, this is indicator they are restricted and when this happens they suck oil into engine cylinder, as crazy as that sounds, it happened to me. Left side exhaust only. I replace all components and hoses, never had an issue since with over 100K miles driven.