Understanding the Three Primary Factors Responsible for Oil Failure

There are three main factors that can cause oil to fail: viscosity, acidity, and contamination. When oil becomes too thin or too thick, it can no longer properly lubricate the engine. Too thin to separate metal parts or too thick to pump.

Similarly, if the oil becomes too acidic, it can corrode engine components and cause damage. Contamination from dirt, debris, or other substances can also cause oil to fail. It is important to regularly monitor and change your oil to ensure optimal engine performance and longevity. However, most oil consumption issues are mechanical in nature and can vary depending on the engine and application.

Too Thin

If the oil in an engine becomes too thin, it can't create a thick enough layer to keep metal surfaces from rubbing against each other. The ideal thickness, or viscosity, of the oil depends on the specific engine design. One important property to consider is the oil's viscosity at 100°C, which can give an indication of whether the oil will provide enough protection when the engine is running at its normal temperature.


When motor oil becomes too thin, it can no longer provide adequate protection to engine parts. One common cause of this is excessive mechanical shear, which can occur as the oil cycles through the engine. This shear stress can weaken the oil's shear strength, particularly in the upper end, piston walls, and bearings of the engine. Over time, this can cause the oil to thin excessively, leaving important engine components vulnerable to damage from metal-to-metal contact. It's important to use motor oil with good shear stability to prevent this from happening.


When motor oil becomes excessively thin, it can be due to fuel contamination in the oil sump. Gasoline and diesel fuel are thinner than motor oil, and when they mix, the oil's viscosity and ability to separate parts are significantly reduced. While a small amount of fuel dilution is common and doesn't have a significant impact on oil life, excessive fuel dilution in mechanically compromised equipment can be very harmful. It's important to monitor oil levels and quality to prevent damage to your engine.

Too Thick

While some may believe that thicker oil is better, when it comes to engine performance, oil that is too thick can be just as harmful as oil that is too thin. Thick oil can cause engine parts to be starved of oil, leading to metal-to-metal contact and potentially causing severe damage to the engine. This is why it's important to use the recommended oil viscosity for your specific vehicle to ensure optimal engine performance and longevity.

When oil is exposed to heat, combustion by-products, and oxygen, a chemical reaction occurs that causes the oil molecules to thicken. This thickened substance, known as sludge, is not as effective at flowing or protecting as the original oil. The chemical reaction that causes this thickening is complex and can be detrimental to the performance of machinery and engines.

If you notice that your oil is becoming too thick and sludgy, it's a sign that it's time to replace it. This can happen when the oil is exposed to high temperatures and low flow for an extended period. While a small amount of varnish is normal, excessive sludge is a clear indication that the oil has degraded beyond repair. To prevent sludge and varnish buildup, it's important to use oil that is resistant to oxidation forces.

AMSOIL synthetic oils are designed with a unique composition of synthetic base oils that contain a higher level of saturated molecules. This composition helps to resist the constant bombardment of oxidation that can occur in traditional oils. In addition, AMSOIL synthetic oils are fortified with antioxidants that work to reduce the formation of free radical oxidation precursors or soak up these precursors once they form, providing superior protection for your engine.

When diesel oils become too thick, it can be due to excessive soot-loading in engines that are not functioning properly. While diesel oil is meant to handle some level of soot contamination, if the soot becomes too much for the oil's dispersants to handle, the oil will thicken. This can lead to excessive wear and tear on diesel liners, which is a common issue seen in engines with high levels of soot buildup.

Too Acidic

When fossil fuels are burned, they produce acids as a by-product. The level of acid formation depends on the type of fuel, engine, and combustion conditions. These acids are carried away by blow-by gases and end up in the motor oil. To prevent engine damage, motor oil contains a detergent that neutralizes these acids before they can accumulate. If the acidity level becomes too high, it can lead to serious engine problems.

When it comes to engine oil, acidity is a major concern. The Total Base Number (TBN) test is used to measure the level of alkalinity in the oil. As the oil ages, the TBN drops, and eventually reaches a critical level where it can no longer neutralize the acids produced during combustion. This can lead to engine damage and decreased performance. It's important to monitor the TBN and change the oil before it becomes too acidic.

If the TBN (Total Base Number) in a system becomes too low, the resulting increase in acidity can lead to rapid corrosion of vulnerable surfaces, such as those made of yellow metals or lined with lead. If left unaddressed, this can cause severe chemical wear and potentially significant damage. While this type of failure is not as common, it is important to monitor and correct TBN levels to prevent it from occurring.

When it comes to the longevity and effectiveness of oil, there are three key factors to consider: viscosity, acidity, and contamination. Fortunately, AMSOIL synthetic motor oils are designed to tackle all three of these issues head-on. By resisting mechanical shear and oxidation-related viscosity increase, while also neutralizing harmful acids and contaminants, AMSOIL oils help ensure that your engine stays protected and performing at its best for as long as possible.

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