Turbo sludge -- I assume it was buried under the oil layers. Now that the oil layers have washed away it can be loosened and help escape the pan.
Since turning DIY, ran my cars on Penzzoil 10W30 conventional with 4-month oil and filter change interval. Past three years, 5W30 Full Synthetic with yearly oil and filter change (7-8k miles) -- cold start during extreme winter morning has improved. Since switching, I find a tea spoon full of fine grain dirt mixed with oil at the bottom of filter housing.

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