Get an OBD II based temperature gauge or any decent scanner and monitor your engine temperatures directly. Ignore the idiot gauge in the dash. I have a Scangauge II I run all the time to do just that. I use the cover of the in-dash CD player to keep it in place.
If your vehicle runs cool I would suspect the thermostat and or the fan clutch.

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2015 X5 xDrive35i MSport
2013 X5 xDrive35i