>>>What do you use to oil bearings?<<<
nothing :- )))

>>>Does dripping thin oil on them really do anything?<<<
yes it does, it makes grease inside thinner

You have to take the shields off and grease them, right?
no, you can put drop of oil also without dismounting shields, but in that case you must use is periodically again and again, orriginal grease is for whole life time, while thinn oil must be refilled after ~50 flights, some makers even sale proper oil, but I did not have luck with that, if bearings do not survive enough with orriginal grease, they usually do not survive anyway, in that case the problem is in construction or mode of use (too small bearings, bad quality, 2 blade prop, resonating rotor at wrong rpm etc)

and there is another problem, some motors have 3 bearings, and it is difficult to reach that inside without complete dismounting ...in that case it is better (beside the best choice not to touch it :-)) ) .. so it is better to replace orriginal grease with high load long life grease, but it usualy costs more than that bearing :- ))) .. you must remove shilding, clean old grease and refill, if you do it carefull you can close shielding back ... in that case you have motor with most expensive bearings (if you count you work time) .. so may be cheaper will be to buy motor with not so cheap bearings :- ))))

It usually goes with price, you cannot expect good bearig used in motor which costs less then high quality bearing :- ))))

If I look around, I see AXI, MVVS, Plettenberg etc can fly several years without oiling and changing. And yes I had troubles with bearing in cheaper motors, I do not think it is good to mention maker, but yes it was cheaper then those 3 

But as I wrote, if you hit resonantion frequency, nothing helps, it will go soon :- )))