They are sealed bearings, but oil can penetrate them. They aren't marine grade, not waterproof bearings. I'm not worried about dirt to be honest, they are sealed in the bottom of the motor mount. If it gets real dirty in there, i have bigger issues I think.
The reason i did this was that I was having a hard time getting them to initiate and would get motor obstructed errors. I ground enough plastic off that shouldn't be causing it, but i couldn't figure it out. I took them back off and just pondered. I kept spinning them and could feel how roughly they were spinning. It reminded me of something .. I spent alot of time rollerblading, skateboarding and now fixing my kids scooters, where old/rusty bearings felt exactly like that. After a quick oil, those things always, always worked better. So i looked up the bearing type, and its MR84ZZ and MR104ZZ .. which are actually pretty common bearings. Used, not only in RC application but other areas .. like skateboards and rollerblades. So i decided that i would oil the bottom bearings and see what happened.
Right here:
After a little oil added and re-mounted, i spun each motor 50 or so times. I could feel the bearings oiled up, loosened up and spinning much more freely. That was really nice. It went from a gritty feeling to a smooth feeling. I can actually spin it and it goes on its own. Since then, my motors all start super easily and no issues