If it was some kind of additive, and there was not much captured in the oil filter then what was in the drained oil must have been heavy enough to settle to the bottom of the oil pan. If it's the bearings going bad, then I'd suspect you would continue to see bearing particles captured in the oil filter even after a few short oil changes. I'd do a few short oil changes with some inexpensive oil of the correct viscosity, and use a new filter every change and inspect the drain pan and filter at each oil change to see if this continues or goes away.