I use a cover made with the NOAH fabric for outdoor use. It's designed for constant sunlight exposure, rain, etc., and has a dust blocker layer. It is also soft on paint. If your X is kept outside, I'd highly recommend switching to a similar type of cover.

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'06 X5 4.8is, TiAg/black, cold weather pkg., comfort seats
'07 750Li, TiAg/black, sport pkg.
'05 Ford F350 silver met./slate gray, Lariat, 6.0 PSD, CC, DRW, FX4