I went 13,000 miles on the factory fill in my first Mercedes.

I thought it was safe because it was a "Mercedes," with "synthetic," and it was what the factory called as appropriate.
Actually, I think it probably didn't hurt anything (it was a fair bit of highway as it was only 9 months). Now my oil changes are very short (3000-5000) so I'm sure it's cleaned up. It might have caused slight more wear from break in metals but I hope it's ok.
If I did it over again, I would have replaced the FF, at half those miles.