I donít know what material is used for clutch disk facing in these Capital gears. Some oil flooded clutches use paper-like fiber material, but some use sintered soft metal. If the latter I would add clutch disk wear to the list of possible sources. The elevated fine particles would point to wear rather than cooler metal corrosion.
Does this gear perhaps have a trolling valve? If so clutch slippage and associated wear may be normal. If it has a trolling valve an external oil filter is often installed, and overlooked when servicing.