Like all professionals, there’s the good, the mediocre & the bad. The good diesel mechanics that I have ran across tried to run things as they were designed as described in a manual somewhere. The only times I seen gear boxes ran slap full were when: 1. Ran by dummies who didn’t know any better. 2. Filled by a dummy who wasn’t paying attention when filling. 3. Operated by a lazy person who didn’t want to fix a leak who wanted to lessen the possibility of it running empty.
If you are responsible for those gear boxes find a manual for them & see what the manufacturer recommends. Also, find the person who filled them to the top & ask them to explain the reason.