This is the total amount of extraneous matter, expressed as a percentage of the quantity of the oil, which is mainly derived from products of combustion blown by the piston rings into the oil wetted areas of the engine. It also includes burnt lubricating oil and additive ash, rust, salt, wear debris and abrasive matter.
Increases may be due to the amount of blow by and sudden increases may indicate piston ring collapse or breakage.
The maximum quantities of insolubles tolerated vary with the type of oil in service. For example, the maximum quantity of total insolubles permitted in an oil with a BN of, say, 9 mg KOH/g would be much lower than that for a higher BN (e.g. 40 KOH/g) heavy duty oil.
The limiting values for insolubles that are set by the engine manufacturers are dependent upon many factors which may include:
- Engine construction
- Operating conditions
- Fuel in use
- Purification facilities
- Size of oil change