Mixing Engine Oil: Is It Saving Money or Silently Damaging Your Engine?

Mixing motor oils is a common practice, often driven by the intention to utilize the leftover half bottle of oil to save costs. Is this a good idea? From a professional standpoint, this seemingly harmless action carries significant risks, which can reduce the performance and gradually wear down your vehicle's engine.

Let’s delve into the unforeseen damages behind this seemingly simple act with the experts at Huynh Chau Lubricants.

1. The hidden dangers of mixing motor oils

When you mix two different types of motor oils into your engine, you're inadvertently creating a chemical battle within your machine. Here are the scientifically proven and practically tested risks involved.

1.1 Additive conflict – The first consequence of mixing motor oils

Each motor oil is a perfectly balanced formula containing a unique system of additives designed to perform functions such as anti-wear, cleaning sludge, and temperature stabilization. When two different oils are mixed, these additive systems may conflict, neutralize each other, or cause unwanted chemical reactions.

For instance, an oil with high alkaline additives can react with another oil that has acidic additives, leading to the formation of insoluble salts. These tiny salt particles can clog oil filters and create deposits within the engine. Imagine this as akin to stones forming in blood vessels, causing blockages and posing risks to the entire system.

Conflicts can also occur at the base oil level, such as when blending synthetic oil with mineral oil. Their heterogeneous molecular structures can significantly reduce the heat resistance and durability of the final mixture.

1.2 Changes in viscosity and performance when mixing motor oils

Many people mistakenly believe that two oil bottles with the same SAE rating, such as 10W-40, can be safely mixed. However, the SAE rating only reflects the viscosity of the oil at a specific temperature. A more important factor is the HTHS rating, which indicates the strength of the oil film at high temperatures and high pressures.

Each oil manufacturer has a unique formulation to achieve the desired HTHS. Mixing oils from two different manufacturers results in a blend with an unpredictable HTHS rating, weakening the protective oil film. Simply put, the oil film becomes more susceptible to tearing when the engine operates under high intensity, increasing the risk of direct wear between metal surfaces like the camshaft and piston.

1.3 Risks of sludge and deposits from mixing two different oils

An experiment by Chevron showed that mixing 30% synthetic oil with 70% mineral oil in a diesel engine resulted in a 45% increase in carbon deposits after just 5,000 km. This