Any one using bearing oil on mini quad motors?
when should we use it?

No need, Motor bearings come shielded (not sealed). Adding oil just attracts dirt and debris. And washes out the grease...don't oil them ever

You can use:
1. High velocity bearing oil like skateboard/rollerblades bearing oil
2. RC car bearing oil
3. Sewing machine oil
4. Automatic transmission oil

The best is to remove the bearings and soak them in oil for some time and remove the oil on the outside with a cloth (be gentle when removing them).

Dust and dirt will get into the bearings anyway and they will be worn out eventually. If your motors survives longer than your bearing, you can get replacement bearings for most motors