During long oil change cycles (usually more than 20,000 kilometers, and up to 100,000 kilometers for some heavy-duty vehicles), the continuous high temperature of the diesel engine, the erosion of the incomplete combustion products of the fuel, and the continuous friction of the mechanical parts will form an extreme test on the stability of the engine oil. The antioxidant and anti-fading capabilities of synthetic diesel engine oil are reflected in this "high-pressure environment" through the stability of the molecular structure, the synergistic effect of additives, and the long-term maintenance of performance indicators, which become its core advantages that distinguish it from mineral oil.
The antioxidant properties of synthetic diesel engine oil first come from the molecular structure advantages of the base oil. Compared with the complex cyclic hydrocarbons and sulfur and nitrogen impurities contained in mineral oil, the molecular chains of synthetic oils (such as PAO polyalphaolefins and ester base oils) are more uniform and regular, and the proportion of unsaturated bonds is extremely low. During long-term high-temperature operation, this molecular structure is not easily attacked by oxygen molecules to cause oxidative cracking. When ordinary mineral oil is above 120°C, unsaturated bonds are easily broken to form aldehydes, ketones and other oxidation products, while synthetic oil can maintain its molecular chain integrity even in the 150-180°C environment common in turbocharged diesel engines, reducing the probability of triggering oxidation reactions from the source.
The synergistic effect of the additive system is the key support for anti-fading ability. Synthetic diesel engine oils are usually compounded with phenolic and amine primary antioxidants and phosphite auxiliary antioxidants: the primary antioxidant interrupts the chain reaction of "oxidation-cracking-reoxidation" by capturing free radicals generated in the oxidation reaction; the auxiliary antioxidant decomposes the peroxide to prevent it from further generating acidic substances. More importantly, the additive solubility of synthetic oil is higher, and it is not easy to fail due to precipitation during long-term use. For example, the retention rate of amine antioxidants in synthetic oil is more than 30% higher than that in mineral oil, ensuring that it can still maintain more than 70% activity after 50,000 kilometers.
Under high temperature and high load conditions, the viscosity stability of synthetic oil directly reflects its anti-fading ability. The temperature of the piston top, turbocharger bearing and other parts of the diesel engine often exceeds 200°C. In this environment, the oil is prone to abnormal viscosity due to oxidation - mineral oil may experience a sudden drop in viscosity due to cracking, or a surge in viscosity due to polymerization to form colloids. However, synthetic oil can control the viscosity change rate within 15% over a long period of time due to its anti-oxidation property: the viscosity index of PAO base oil generally exceeds 140, and even after 100,000 kilometers of operation, it can still maintain the lubrication efficiency of the original viscosity grade, ensuring that the oil film strength meets the friction requirements of the piston and cylinder liner, camshaft and tappet.
The ability to control sludge and sediment is a "visual manifestation" of anti-oxidation. During long-term use, oil oxidation will generate sludge (a mixture of soluble colloids and carbon particles) and varnish (a high-temperature oxidation product), which will block the oil passages and increase wear. The detergent dispersant of synthetic diesel engine oil can wrap and disperse the tiny particles produced by oxidation to prevent them from agglomerating and forming sludge; at the same time, the low volatility of its base oil (the evaporation loss rate is more than 50% lower than that of mineral oil) reduces the heavy oxidation products remaining after the evaporation of the light components, so that the engine interior remains clean after a long period of time. This feature is particularly evident in the long-distance continuous operation of heavy trucks.
The slow change of acid value and alkalinity value confirms the persistence of anti-fading. Sulfur oxides and nitrogen oxides produced by diesel combustion will combine with water to form acidic substances, which will erode the alkaline reserves of the engine oil (measured by total base number TBN). Synthetic oil has strong antioxidant properties, less organic acid generated by self-oxidation, and the alkaline components in additives (such as sulfonates) are released more slowly. Data shows that the TBN of high-quality synthetic diesel engine oil can still maintain more than 50% of the initial value after 80,000 kilometers, and the acid value (TAN) growth rate is 40% slower than that of mineral oil, avoiding corrosion of metal parts and oil film failure caused by excessive acidity.
Compatibility with rubber seals is also an implicit manifestation of anti-fading. During long-term use, if the engine oil is severely oxidized, its polar substances will corrode the rubber oil seal, causing the seal to fail. Synthetic oil has fewer oxidation products and higher chemical stability, and is more compatible with fluororubber and nitrile rubber. It will not cause the oil seal to harden or swell within a 100,000-kilometer cycle, which indirectly ensures that the engine oil does not leak or be contaminated, forming a virtuous cycle of performance maintenance.
Ultimately, the stability of engine power output and fuel economy is a comprehensive verification of antioxidant and anti-fading capabilities. When the engine oil decays due to oxidation, the lubrication resistance increases, the fuel combustion efficiency decreases, and the vehicle will experience power weakness and increased fuel consumption. For vehicles using synthetic diesel engine oil, the power attenuation can be controlled within 5% during a long oil change cycle, and the fuel consumption fluctuation per 100 kilometers does not exceed 0.3 liters. This is the result of the continuous effect of its antioxidant and anti-fading capabilities, and it is also the core value of long-term use.
