At work we have an environmental chamber with cascade cooling (R404/R23). Recently the
high side compressor would trip out on thermal overload when the system was operating with a high load (hot & controlling high humidity). The Technician evaluated the situation & we discussed options. In the end, it appeared to be a restriction so all the high side cap tubes/dryers were replaced along with the compressor. After flushing the system with pentane, the residue included some "grainy" stuff & a bit of "goop". The Technician says he has seen "goop" with POE lubricants in these sorts of systems (they do a lot of ultracold work). It turns out I could tap one of the filter/dryers from upstream of a cap tube on the bench and get gritty stuff to fall out. DUH - this was from the "in" direction and I disposed of the filter/dryer/cap tube before figuring this out. I should have sliced the filter/dryer open and inspected the insides to see if the filter media was disintegrating.

I did capture some oil from the removed compressor and when allowed to settle there is a dark layer of crud on the bottom of the vial.

Question is, why would there be ANY grainy stuff in there? This isn't a cheap off-brand system.