Hey,
I just bought an older 350 Moto 4, was doing and an oil change earlier, completed that and moved onto the rear final drive case or (differential as some might say).

I unscrewed the drain plug and the oily substance that came out seemed to be a silver/gray color, it may have had very small pieces of metal in it? there were like microscopic pieces of glitter or something.... I let it all drain and poured some SAE 80w90 hypoid lube in, let it flow through in order to take the rest of the nasty oil silver crap out with it. Once it seemed to be clear oil again I put the plug back in and filled the reservoir back up.

What do you think, that silver crud was?

 

That is metal shavings from wear to the drive gears. This is common as most people don't change read end oil so it tends to build up. If there were no large pieces of metal just flakes then this should not be to worry some. If maintained from now on I would hazard to say you can expect no problems.