Can be used to determine the serviceability of the lubricant.
The acid number is the quantity of acid or acid-like components in a new or used lubricating oil.
A new oil may have some acidic constituent as part of its additive package designed to specifically enhance its performance. Increases in the acid number may be due to oxidation, nitration, or through contamination with some acidic type product. The acid number can be used to determine the serviceability of the lubricant in specific applications such as refrigeration and air compressor equipment.
| Causes | Effects |
| Overheating | Corrosion of metalic components |
| Excessive blow-by | Promotes oxidation |
| Over extened drain intervals | Oil degradation |
| Improper oil type | Causes additive depletion |
| Recommendations |
| Drain oil |
| Reduce oil drain interval or use synthetic lubricants |
| Verify correct oil type for service |
| Check operating temperatures |
| Check overheating |
