In mineral oils, deposits and soluble sludge represent impurities and oxidation products formed during the life of the oil. These components can form as a result of oil aging, thermal degradation and oxidation. The determination of sludge and soluble sludge provides valuable information about an oil's condition, performance and replacement requirement.

**ASTM D893**: This standard includes the procedure for determining the amount of sediment and soluble sludge in oils.

**Application of the Test**:

1. **Sample Preparation**: A specified amount of oil is mixed with a solvent. This separates the sludge and soluble sludge while dissolving the oil's base oil and additives.

2. **Centrifuge**: The mixture is centrifuged for a specified time and speed. This process ensures that solid particles and soluble sludge are separated from the mixture.

3. **Evaluation**: After centrifugation, the volume of the precipitated material is measured and calculated as a percentage. This represents the amount of sediment and soluble sludge in the oil.

This test is used to evaluate the oxidation level and lifespan of the oil. A high amount of sediment and soluble sludge may indicate that the oil is over-aged, oxidized or contaminated. Considering that this type of oil can adversely affect engine performance and component life, this type of analysis is very important. If testing reveals high levels of deposits or soluble sludge, it is recommended to change the oil.