The bearings are not sealed, just shielded, which means they only keep the big stuff out, dust easily can affect them.
The proper way to service the bearings is to take apart the motor, remove the bearings, take the clips off the shields, remove shields. Clean all parts, then put a very slight dab of grease, reassemble. If I need to take apart the motor, it's far easier simply to replace the bearings, so using a light machine oil is the quickest fix and will keep you flying.
As for me I use a needle oiler and put a drop of oil in each bearing if the motors start to feel warmer than usual. I had a new set of T-motors starting to seize because I had launched from a dirt field. All I did was lube them and they've been fine since.
As for my credentials....much too long to list.....however I have taken apart the motors on my Phantom after having one seize after less than 10 flights. Guess you can say they are sealed for life, since I chose to replace the bearings instead of taking them apart, in my case life was just a handful of flights.....want to extend their life, lube them regularly.
https://www.youtube.com/watch?v=8UVu8YuC8iE
https://www.youtube.com/watch?v=UYqtwd4xXYo