XK150 Which oil is used in the T5?

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Tremec recommends Dexron II. Dexron is a General Motors trademark. See Tremec T5 service manual

Quantity: 2.3 litres (4.8 US pints)

On the left of the photo is fresh Dex III (hard to find Dex II these days). On the right, Dex III after 800 km (500 mile) road use in my T5 (odometer reading 8038 mi). There are fine soft solid particles in the used sample. The particles remain mostly in suspension. None of the particles is attracted to a magnet.

What is the source of the particles? What will the drained oil look like at the next change?

Update - November 2016:

In August 2016, the oil was drained from the T5 with the odometer reading 9659 mi. There was some, but only a minimal amount of the non-metallic particle matter in the drained oil. New Dex III was used to refill the gearbox.