This is part of the oil check valve, or oil drain-back valve, depending on the terminology you use. When you stop the engine, it stops the oil from draining back into the sump. This means that oil is already there when you restart the engine, it doesn't have to be pumped up. Many filter designs have this valve inside the filter. BMW puts it in the oil filter reservoir. I remember a campaign on replacing it years back. I also know that more than one person who found the pieces did nothing except remove them, and disaster did not follow.

Here is one thread:

http://www.xoutpost.com/bmw-sav-foru...il-filter.html

Here is another one, with a resolution history.

http://www.xoutpost.com/bmw-sav-foru...ce-update.html

Jeff

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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