Originally Posted by
davidad
The crud on the chain comes from the inside in the form of metal wear and oil.
Not quite. While some of that is wear by product, the vast majority is road dust.
There simply isn't that much wear byproduct, and it's in the form of superfine dust and in any case wouldn't migrate to the outside of the plates. One glance at a chain and it's obvious that what's glued to the outside isn't superfine metal dust.