Can oil prevent corrosion on the inside of the engine?

Acid Resistance

Resistance to acids is a measure of an oil's ability to neutralize harmful acids in the engine. During operation, the engine produces fuel combustion by-products which form strong acids. As engine oil oxidizes, it also forms acids of a weaker type. Unless neutralized, these acids can damage an engine by attacking engine components exposed to the oil.

Engine oils are formulated to contain additives, known as bases, which neutralize acids. An oil's level of acid resistance is determined by measuring the amount of acid that can be neutralized by the bases in an oil. This is done by using ASTM test method D2896. The oil is mixed with increasing amounts of an acid solution until the bases in the oil are overcome. The amount of base additive that is present in the oil can then be calculated to determine the acid-neutralizing value called the Base Number, expressed as milligrams per gram of oil (mg/g).

No specifications have been set for acceptable Base Number limits. Up to a point, the higher the Base Number, the more ability an oil has to protect the engine from acids. However, if the Base Number is too high for the particular engine type or design, excessive ash deposits may form during oil combustion which can cause abrasive wear of the cylinder walls. In general, the Base Number for gasoline engine oils should range from 6 to 12 mg/g. Base Numbers below 6 mg/g signal a need to change the oil more frequently to prevent acid buildup, particularly with short-trip driving patterns.

Base Number measurements are frequently used for fleet monitoring to predict the useful remaining life of the engine oil by determining the change in Base Number as an oil is used in the engine.