Front differential metal shavings.

If you have to "top it off" it has a leak

 

Have you had a oilsample analysed?

 

Myself, I'd probably do as was mentioned earlier and plug swap it a few times and see if anything else turns up.

 

Obviously lol
I rarely see a dry differential on old trucks.That’s very common. When you you have a small leak around those covers, you kind just leave it alone and check every year or so. The type of leak that you see a few drops on the ground I’m the morning

 

I would start with sending oilsamples .............it will show the amount off wear without guessing.

 

After awhile in the hot climate down here they will lose a little. It will vapor out through the breathers over time. That is why most guys down here will overfill a little.

 

If I remember correctly I thought the change interval for synthetic was 500,000 miles.

 

I had a little leak in one of my differentials that when I really cleaned it up and looked at it one of the bolts was loose.

So I tightened that one and I went over all of them and fortunately no more leak.