My 2003 E320 Wagon is low on hydraulic power steering fluid (the tank beside the windshield washer fluid tank,on the driver's side). I see the proper fluid is Fuchs Titan,#001 989 20 03. To bring it up over the "min" line,can I simply add to what's there or should I drain,flush and refill?? I'm hoping to just add fluid to what is already there. Any advice?? Thanks.

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2001 E320 4matic, 2003 E320 4matic wagon