If the oil in the engine is clean (ie, you haven't driven it at all) you could leave it longer without any worries. That is the definition of shelf life, to me. If you have been driving it, but not much, and it gets to a year but has very low mileage it is really your call. The oil is contaminated somewhat. Manufacturers aren't recommending annual oil changes like they used to. It has long been considered good practice to change it annually at a minimum, and many enthusiasts will do it annually on principle. Personally, I now let mine go more than a year, and my usual change is closer to 18 months. I wouldn't leave it much longer than that.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue