Even after an overhaul, my SM420 always had a little bit of gear noise, but not objectionable.
It's had 90w hypoid gear oil in it since before I owned it (I changed it back in the '90s and drained old sulfur out) so shoot, I thought that was what they were supposed to have!
When I downshift from third to second, there's always a little whine as the synchro catches up, I just don't force it and then it falls right into gear. Just like how Dad drove his (same make model year and drivetrain) dump truck. That downshifting synchro sound meant we were turning up the street to our house and were almost home.
