My gap year, I worked at a Shell service station. Shell Manuka to be precise.
The two oils that we sold were Shell Super, 20W40, and XMO which was a 15W30...if only I knew then what I know now....then XMO HI (Hi vis, a 40 grade came in with XHVI basestocks).
I had access to the 40/50ml of oil that was left in every litre bottle that was left in the bins, and quickly came up with full litres. Used them for top-up.
The white bottles, (plus Dad's GTX) always had a residue in the bottom that was very dark grey, nearly black...never sandy or dirt looking.
Former member Stinky Peterson did all the VOAs with particle count, and virgin oils were quite putrid. Here's a couple from my hard drive.
At work, the turbine bypass system needs to be filtered to 5um before we add the oil, or it will never clean up in service.
When Blackstone tested the old ebay oils, they took a can, and put it in a paint shaker versus just testing another. They had no difference between the thoroughly mixed and the stagnant oil WRT additive concentrations.
Leads me to think that it's just dirt.