Rear diff oil grey and muddy coloured

Two things I have.

Thing One: I changed my diff oil myself for the first time and the rear was grey coloured with the occasional strand of black in it. I’ll put a picture up. The front was green and looked like new so I’m wondering if there’s a leak in the back and it’s filling with crud. My dealers closed or I’d call them but I’m wondering if anyone with a bad seal ever had the same looking fluid when they drained it.

Thing Two: I grabbed the wrong bottle off the bench and put 75w 90 in the gearbox instead of 140. Can I run at least a while or should I drain it and put the 140 in. That drain bolt is a BITCH.