Originally Posted by JCL

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.