Honestly, nothing bad will happen if you put 87 in your X5. Both the M62 and N62 have knock sensors and can adjust the timing based on the octane of fuel that you use. You might lose a little bit of power, but you won't actually harm your engine.

Heck, I even ran 87 in my 2000 M5 one time on a road trip and everything was fine. And that engine is a lot more exotic than an M62 with a higher compression ratio, and yet it was fine. BMW engineered these engines really well!

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2003 X5 4.6 | 2000 540i Touring

Formerly owned:
2004 Range Rover | 2000 M5 | 2000 540i/6
1995 750il | 2003 Mercedes S500 | 2003 540i/6