We may still be arguing semantics, but I don't think of the natural wear products within the transmission as contaminants. They are designed to be in there. The fluid doesn't get chemically changed, for example, like engine oil does. It certainly does get thicker due to clutch wear material, but fortunately the transmission adapts to that viscosity change, by design. It works fine until the filter is clogged and pressures drop. As we have seen, though, it can go hundreds of thousands of miles without that happening.

I would say the most common source of contamination is the fill plug, when owners introduce non-spec fluids.

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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