The solids test measures the amount of dissolved or suspended material that can be centrifuged out of the used oil when diluted with a solvent. Included in this material are

• Fuel Soot
• Wear particles
• Products of oxidation and nitration
• Dirt and other environmental debris

Fuel soot is usually the main constituent of the total solids for diesel engines. It is formed during the combustion process and relates directly to combustion efficiency. Poor efficiency of combustion creates high levels of fuel soot while good combustion efficiency creates relatively low levels. Acceptable levels of fuel soot are determined by make and model of the engine. Higher levels are attributed to improper air to fuel ratios, worn or stuck rings, high temperature operation, lugging and over-extended drain intervals. Infrared analysis is a method of determining the amount of fuel soot present in total solid content. Usually, routine analysis does not include this test unless very high levels of solids are indicated.

Wear particles
Particles of engine wear metals are usually a minor part of the total solids and are better quantified by spectro-chemical analysis. This test will be discussed later.

Products of oxidation and nitration
Oxidation and nitration are chemical reactions that change the oil’s composition. Typically, engine oils are formulated with chemical additives which inhibit these reactions. However, overheating, extended drains, excessive blow by use of high Sulfur fuels can deplete these additives and allow the reactions to occur, the end result of which includes excessive oil thickening, metal corrosion and lacquer formation. The test used to measure degrees of oxidation and nitration is once again infrared analysis, which is run only when a severe condition is detected by normal solids testing.

Dirt and other environmental debris
Dirt content is usually a small percentage of total solids and is quantified by spectro-chemical analysis of Silicon.