I don't think you get a "fluid level" warning....i haven't gotten any and i know i have a leak. However, depending on the severity of the leak you'd notice that the steering is getting heavy and the gearbox shifts roughly....worst case it would go into safe mode.

BMW X5 4.8iS 2005

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BMW X5 E53, 4.8iS - 2005
Professional Stereo/Nav with DSP (modified BM54 module) - Converted to Eonon GA5166X with Reverse cam
4-Corner Air Suspension - Converted to BC Coilovers