Lubricating oils are used to cover or treat mechanical components so as to lessen friction. 
Lubricating oils has number of important functions in the diesel engine.Some advantages of lubricating oil
- Wear reduction of components such as bearings
- Reducing friction of boundary lubricated components
- Piston cooling methods
- Wear reduction of pistons and piston rings
- Corrosion prevention due to acids and moisture
- Prevent sludge build-up on internal surfaces
- Serving as a hydraulic media
- Increasing the performance and life of engine