Kia Troubleshooting Questions? Ask a Mechanic
All engines will sludge up to some extent these days, depends on many factors.
Your mechanic believes sludge and metallic is not related to his work.
I think that the catastrophic loss of oil caused the high metallic particles.
For the engine to have an oil change, and then have a failure like this can only be attributed to oil starvation.
Strange that the oil pressure warning lamp did not come on to warn you.
There is just no excuse for leaving a sump plug loose, period.
Small claims court is your next action.
I hope my advice has been of assistance to you.
Have you made any progress with your problem?