Alkaline additives are used to neutralise the acidic components that can form during the combustion process. The additives also offer other performance- enhancing properties, including detergency. Base numbers place a pre- determined value on the amount of alkaline additives.
Engine oils usually begin with a relatively high base number which decreases over time. Occasionally, base numbers can also be used to establish oil drain intervals and are primarily used in monitoring the condition of marine diesel engine oils where very high sulphur fuels are often used.
| Causes | Effects |
| Overheating | Increased acid number |
| Over extended oil drain | Oil degradation |
| Improper oil type | Corrosion of metal parts |
| High sulphur fuel | Increased wear |
| Recommendations |
| Use low sulphur fuel |
| Follow manufacturer's recommended drain interval |
| Check quality of fuel |
| Use correct oil type |
| Decrease oil drain interval if used in severe service |
