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Oils do not wear out. That is to say, the base oil does not generally deteriorate. However, oil gradually loses its ability to carry out its functions of lubricating, reducing friction and dissipating heat.
This gradual loss of lubrication quality is caused by self‑generated contamination of components within the oil, oxidation due to overheating, viscosity changes caused by fuel dilution or oxidation, water entry into the system (either directly or through condensation) and the gradual depletion of the additive package, as the additives counteract acids and disperse and suspend foreign materials.
There are other conditions that also contribute to oil degradation to a lesser degree, such as poor-quality fuel, operation of equipment in over-cooled conditions, lean air/fuel ratios and emission control systems that are malfunctioning or have been removed or disconnected.
The conditions that contribute to oil degradation and necessitate regular oil drains are described in detail below:
Every engine, hydraulic pump, gearset or other component gradually wears as it operates. Tiny, sub-micronic particles of elements such as iron and copper become catalysts that slowly attack the oil, causing acids to form.
These tiny metal contaminants, combined with carbon soot particles resulting from the combustion process in engines, also circulate in the system and through their abrasive action create more wear. These solid particles become just like a fine-grinding compound and will slowly scratch and score bearing surfaces, turbocharger bearings, crankshaft journals, cylinder liners and hydraulic pump and valve surfaces.
Normal full‑flow filters are rated at about 10 microns in hydraulic systems and 40 microns in engines and will not remove these sub‑micronic particles. Furthermore, if the filters become plugged or if the bypass valves within them remain open for long periods of time, such as at cold startup, contaminated oil will be pumped throughout the system.
It is important to remember that if the levels of self‑generated contamination are allowed to increase until some damage by abrasion is caused, it will be too late to prevent more on-going damage, even if the dirty oil is drained.
Oxidation is defined as a chemical deterioration of an oil that is the result of continued contact with oxygen and catalysts such as copper. Oxidation causes the oil to thicken or increase in viscosity. This leads to reduced oil flow and reduced heat dissipation, which in turn accelerates the oxidation process.
Any practice that increases the operating temperatures of an engine, gear drive or hydraulic oils must be avoided. Oil sump temperatures in engines operating at high speed and under heavy loads can reach or exceed 150 degrees C (302 degrees F), with piston ring temperatures exceeding 310 degrees C (590 degrees F). Turbochargers may operate at up to 100,000 rpm with oil temperatures exceeding 315 degrees C (600 degrees F).
If engines or turbines are shut down immediately after operating at these temperatures, oxidation degradation of the oil can take place very quickly. Furthermore, if extremely hot oil is allowed to remain in the turbocharger or turbine bearing housings on hot shutdowns, oxidation can occur so rapidly that the oil may actually "coke" into a tar‑like or carbon substance, which will plug turbocharger and turbine bearing oil passages.
Inoperative emission controls, retarded spark timing and thermostats with too high opening temperatures will increase the thermal load on gasoline engines, in turn increasing the rate of oxidation. In addition, improperly adjusted air/fuel ratios contribute to oxidative oil thickening because they produce blow-by gases containing high concentrations of nitrogen-dioxide compounds that rapidly promote oil thickening.
Hydraulic systems, on the other hand, operate best when the bulk oil in the reservoir does not exceed 60 degrees C (140 degrees F). Oxidation rates within hydraulic oils will approximately double for every 10 degrees C (18 degrees F) increase in temperature. Continued oxidation of oil will cause sludge and varnish deposits to form and acid buildup to cause corrosion.
Any water entry into hydraulic or turbine systems can cause severe damage, particularly in systems with very close tolerances, such as hydrostatic transmission systems that use piston pumps.
Water contamination caused by condensation is a common problem in North American climates due to dramatic changes in temperatures.
Hydraulic or turbine oil with a "milky" appearance is an indication of water in excess of 2,000 ppm, prompting action to be taken to replace the oil or remove the water through special filtration systems that are available for this purpose.
In gasoline engines, the combustion of 1 gallon of gasoline produces about 1 gallon of water. Any moisture entering the crankcase through "blow-by" can cause rusting, corrosion, sludge deposits and the separation of additives from the base oil.
Water entering the oil in a diesel engine can also be extremely harmful, particularly if poor-quality fuel is used. Low-quality fuel is generally high in sulfur content, and when water combines with sulfur dioxide, acids are formed, which will immediately attack bearing surfaces.
When base oils are formulated and blended with additives, these elements are slowly "used up" in performing their functions. Anti‑wear and extreme-pressure agents are depleted as they are deposited on metal surfaces.
Detergents and dispersants are used up as they continue to counteract various contaminant particles in the oil. Additives that protect against acid attacks are depleted as they counteract acid formations within the oil. The additive package will eventually become depleted or ineffective if the oil is allowed to remain in service for extended periods.
As noted previously, the use of low-quality diesel fuel can cause severe corrosion problems due to the formation of acids resulting from high sulfur content. If, however, it is necessary to use fuels with more than 0.5-percent sulfur content, a lubricating oil with a minimum API service rating of CG-4 should be used. These oils generally are better able to counteract acids than oils of lesser quality.
Today, the more serious problem is premature wear of fuel system components due to the poor lubricity of diesel fuel as a result of the removal of the natural lubricant sulfur. Ensure that water jacket temperatures are generally maintained above 79.5 degrees C (175 degrees F) to minimize sulfur attack.
This means never operating an engine without a thermostat or with the wrong thermostat. It also requires that excessive idling of diesel engines must be avoided.
Unleaded gasoline is recommended for all gasoline engines not only to reduce emissions but also to reduce the formation of lead compounds that form deposits and can contribute to self‑generating contaminant wear within the engine.
The conditions described above can be prevented by:
Read more on best practices for lubricants:
How to Evaluate a New Lubricant
Are Higher-Priced Lubricants Better?
When Is It Hot Enough for a Synthetic?