What kind of differential damage have you seen due to wrong gear oil . . .?

Close, but not exactly . . .

The shop that it was sublet to installed a complete NEW axle assembly . . . not a used one from a junkyard or some other kind of donor vehicle

Your way was cheap . . .

I didn’t see the invoice for this particular repair . . . but I can guarantee it was anything BUT cheap

Oh, I know that had a new one. Just talking about swapping it out.