Just got the same thing last night actually. Have two vids to prove it. The last time this happened I went out and bought Pennzoil Snyncromesh, which didn't seem to help out as much as I was expecting. I drained that after a month, then went out and bought GM synchromesh Friction Modified (now discontinued), and it cleared up almost all my grinding (bandaid but it works). Haven't changed it since, although I probably should have.
I'll be picking up the AC Delco brand Synchromesh Friction Modified (AC Delco part #10-4014) later today or tomorrow. Hopefully that will help with the grinding at high rpms.