The best tip but there is more so here is one.

the easiest way is after filling gear oil properly first - let all of it sit for a bit the re-take out the vent plug screw wait for all air to purge & let a solid constant stream of gear oil flow out - reinstall the vent plug screw - then recheck level in the gear oil reservoir bottle. Fill the reservoir bottle if needed to the proper level with gear oil . Stress the word VENT PLUG SCREW as one of the purposes it does.