I purchased my 2011 X5 xDrive35d 25 November 2014 with 72616 miles on it.

12/17/2014, 74124 miles: DEF No Start warning
12/17/2014, 74174: approximately 40 ozs added to the Active tank; the warning cleared after about 100 feet of leaving the parking spot
1/1/2015, 74552: Low DEF warning
1/1/2015, 74557: filled with remaining 1/2 gallon bottle of BMW DEF, ~20 ozs
1/2/2015, 74600: added ~one gallon to Active tank and 1/2 gallon to Passive tank
1/24/2015, 75213: added ~0.25 gallon to Active tank and 2.5 gallons to Passive tank; levels were visible in both fill openings
10/13/2015, 85219: Low DEF warning
10/16/2015, 85498: added one gallon to Active tank and four gallons to Passive tank

I'm comfortable with the SCR system performance. I believe the literature states a nominal 10000 miles between warnings and I seem to be along that curve. Driving conditions over these nine months and 10000 miles included several months of very cold weather, a few minutes of cold idling in the morning, several weeks of commutes that were approximately 15 or 30 miles round trip an average of six days a week, some months of hot weather with identical commutes, some driving around town in all types of weather, some very short round trips of less than three miles and a few hours total of idling in very hot weather.

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8/2011 X5 xDrive35d Sport, Black Sapphire Metallic, Black Nevada Leather, Fine Burr Walnut Trim
2/2001 M5, Jet Black, Exclusive Complete Black Walk Nappa Point Heritage Leather, Black Cubic Trim