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Mixing up motor oils — Diesel vs. Gasoline explained
Gasoline engines run cleaner than diesel engines, which produce significantly more soot, have a higher compression ratio, and generate more intense heat. These differences determine the need for specialized motor oils.
Jalopnik wrote about this.
Big difference
In diesel engines, the fuel combustion process occurs with the help of strong air compression, which is much more intense than in gasoline engines. This high pressure leads to increased temperatures and the formation of large amounts of soot. If left unchecked, the soot can settle on the internal surfaces of the engine and cause accelerated wear.
Diesel engine oils are produced with more powerful detergents and additives, such as Zinc dialkyldithiophosphate (ZDDP), to combat wear and corrosion. Their higher total base number (TBN) neutralizes the accumulation of acids from combustion byproducts and remains effective much longer than gasoline oils do.
Additionally, the high-pressure environment of a diesel engine requires oil with shear stability. Strong polymers are used to maintain viscosity under increased loads. Diesel oil formulas contain low-ash additives that protect diesel particulate filters (DPFs) and catalytic converters.
Gasoline motor oil has a lighter texture and is optimized for cleaner-running engines that operate at higher RPMs but are subject to less torque stress. It has lower viscosity, which is designed to promote cleaner combustion and better fuel economy. It does not require strong acid resistance or soot control.
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Which oil should you use?
It is always necessary to use oil specifically designed for diesel engines — preferably with high TBN and low ash content. Failure to do so can result in clogged soot filters, acid corrosion, and severe camshaft or bearing wear after only a few thousand kilometers. Even if gasoline oil meets API specifications, it lacks the chemical reinforcement necessary for diesel fuel combustion dynamics.
Some modern oils have dual labeling, such as CK-4/SN or 5W-40, and are designed for use in both diesel and gasoline engines.
However, you should always carefully check your car's manufacturer requirements before using such universal mixtures. Using the wrong oil can lead to expensive consequences.
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