Contaminant Particle Sizes

Anything in your oil that was not part of the original formulation is considered a contaminant.  Some contaminants are fluid, such as water or fuel, while others are molecular, such as the acids produced within the engine or residual additives left over from a previous lubricant.

While water, fuel, acids and residual additives are important to identify when using oil analysis, it is hard particles that are most often misunderstood. Take soot, for example. Soot is formed during diesel fuel combustion and can consist of a variety of particle sizes. Most soot particles are smaller than one micron. These particles are suspended harmlessly in the oil and are too small to be removed by any filters.  They are the reason diesel oil often turns black during use. If these small particles begin to agglomerate, or stick together to form larger particles, typical filters can then remove them before they can cause wear to the engine.  In an oil analysis, soot is measured as a percentage of volume because too many soot particles of any size will eventually increase the oils viscosity of the oil.

Wear-metal particles — iron, chromium, nickel, aluminum, copper, lead, tin, cadmium, silver and vanadium — also come in a variety of sizes. The wear-metal particles reported in a regular oil analysis will be less than 5 to 8 microns in size since that is the upper limit for particles caused by normal wear conditions. Wear particles larger than 8 microns found in the engine oil suggest a mechanical condition in the engine. Occasionally, a customer will be able to see metal particles in an oil sample and still receive normal test results. This is because particles that large are only caused by an existing mechanical problem, such as a loose or misaligned component, which can’t be fixed with motor oil.