working on my rear disk brake swap, there is a layer of grey sludge on diff cover magnet. pretty much covered the magnet. how much is too much?

 

why did you pull the magnet? Are you going to replace the fluid?

It's pretty common. Unless it seems to build at an alarming rate or seems like chunks instead of just fine metal shavings... I wouldn't get worried too much. As long as the diff still acts normal, sounds normal, seems normal, then its probably 'normal'. Hard to judge without a picture but I would suspect everything is fine because you aren't like "I found a chunk of metal" or whatever the post-mortem question phrasing

 

diff is from 1991 camaro super car. don’t know mileage. refreshing it before it is installed into my 90.

 

Don't sweat it. It's most likely normal. Unless the glob is the size of a golf ball.........