Both oils and greases have their advantages and we’ll forgive you for using the terms interchangeably. Technically though, grease is defined as an oil mixed with a thickener. Because of this, oils tend to provide higher running speeds, but will not stay in place as effectively. Therefore, continuous re-application may be needed whether the bearing is sealed or not.
For many applications, grease is more convenient as it stays inside the bearing for longer. It benefits from both the properties of the base oil and the added thickener; both having positive effects on the performance of the bearing and its lifespan.
When choosing between oils or greases, it’s important to remember frictional torque levels for a greased bearing are briefly higher to start with, as the bearing begins to spin from stationary. This is because thicker grease takes a short time to ‘run in’ and get distributed inside the bearing. It’s most likely not a problem for many continuous applications, but this could cause problems for stop start equipment.