Why might I have to add oil between changes?

Volatility

Depending on temperature, harsh mixing, and the amount of engine oil entering the combustion chamber, engine oil can evaporate and leave the engine. This is called volatility and is a measurement of the amount of oil which is lost during engine operation.

There are several concerns associated with oil volatility. First of all, the lost engine oil must be replaced. Additionally, since the most volatile components of the engine oil are the first to leave the engine, the viscosity of the remaining oil increases. The chemical properties and additives of the oil also change as the volatile components leave the oil. Finally, there is evidence that the volatilized oil may damage the catalytic converter as it passes through.

The volatility of an engine oil is measured using ASTM test method D5800. A known weight of oil is heated to 250°C in a special chamber and held at that temperature for one hour. Air is introduced into the chamber and maintained at a constant flow rate under slight vacuum. After one hour, the amount of oil remaining in the chamber is weighed again. The percentage of oil lost is determined by comparing the remaining weight of oil with the original weight of oil.

Specifications set by ILSAC determine that the amount of engine oil lost through volatilization at 250°C (482°F) for one hour should not exceed 15%.