Always in.
But, if you gotta leave it out, a short amount of time isn't going to hurt anything. Especially is you Zaino.
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Current - 2011 N55/DCT E82 135i--Lemans Blue, Gray Leather, M Sport, Premium
Past BMWs: 2008 E82 135 - AW, N54, MT
2007 E92 328 - JB, N52, MT, ZSP (traded in for 135)
2007 E92 328 - JB, N52, MT (retired by a drunk driver)