If a bike bearing has oil on it and needs lubrication, it is generally not recommended to replace the oil with grease. Oil and grease have different properties and uses when it comes to bike bearings.
Oil is typically used for bike bearings because it has low viscosity, allowing it to flow more freely and reach all parts of the bearing. It provides excellent lubrication and helps reduce friction. The downside of using oil is that it tends to have a shorter lifespan and may require more frequent reapplication.
On the other hand, grease is a semi-solid lubricant composed of oil and a thickening agent. It has higher viscosity than oil, which means it stays in place and adheres to the bearing surfaces more effectively. Grease provides longer-lasting lubrication but can be less effective at reducing friction compared to oil, especially in high-speed applications.
If the bike bearing is designed to be lubricated with oil, it is generally best to stick with oil for optimal performance. Using grease instead of oil can lead to increased friction, decreased efficiency, and potential damage to the bearing. However, it's important to consult the manufacturer's guidelines or seek advice from a professional bike mechanic to ensure you use the appropriate lubricant for your specific bike and bearing type.