So far changed my differential fluid twice at about 25K each time. First time around there was a thick grease sludge on the magnet drain plug. Second change still had it just not as much as the first time.

I was curious will the drain plug ever come out clean or will there always be a certain amount of sludge in the differential because the amount of miles that the gear oil undergoes compared to say engine oil?
Also does it matter if it sludges a little? Since the differential is sealed it doesnt need to circulate the oil, its just providing a nice pool for the gears from how I see it.
Looking for thoughts, advice, opinions.

 

The sludge is the fluid mixed with pulverized metal. Shouldnt be too much.
Perfectly normal, gears wear over time, no matter how good the lube is.