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Three differences between passenger car motor oils and heavy-duty motor oils

Motor Oil

Have you ever been curious about what the difference is between the oil required to keep a passenger car’s motor running smoothly and heavy-duty solution for use in larger vehicles like trucks and other freight vehicles? Many people often ask if these two types of motor oil are interchangeable and wonder if they are suitable for use in the different types of engines that are installed in each vehicle and what the potential impact might be.

In this blog, we’ll run through the major differences between these two different types of motor oil and look at the effect of using these solutions in engines other than the ones they were designed for. Read on to find out more.

Anti-wear loads

To understand the first difference between the two motor oils, we must look at catalytic converters.

Catalytic converters are large metal boxes that are bolted on to the underside of vehicles. They have two pipes that extrude from them. One is for the “output” and the other for “input”. The input pipe connects to the vehicle engine and brings in fumes from the cylinder head that are hot and polluted. The output pipe connects to the tailpipe. Gases from the engine fumes bypass the catalyst and chemical reactions take place, breaking apart pollutant gases and then converting them into safer gases that are blown relatively harmlessly into the atmosphere.

Typically, each converter has two catalysts. One manages nitrogen-oxide pollution by removing oxygen. As a result, nitrogen oxides are broken down into nitrogen and oxygen. These gases are essentially harmless as they are already naturally present in air. The second catalyst adds oxygen and handles the carbon monoxide, turning it into carbon dioxide. It also converts any unburned hydrocarbons in exhaust fumes into water and carbon dioxide. After the catalytic converter has performed its function, the exhaust emits oxygen, nitrogen and carbon dioxide, along with water in steam form.

Many of the by-products of combustion – namely lead, phosphorus, zinc and sulphur – can severely impact the converter’s ability to carry out its role. To cope with this, diesel motor oils have a far higher anti-wear load that takes the form of zinc ZDDP (dialkyldithiophosphate). As a result, catalytic converters used in diesel systems are engineered to handle this additive, while petrol systems used in passenger cars are not. This is the first significant difference between passenger car motor oils and diesel motor oil and one of the crucial reasons that you should never use diesel motor oil in your passenger car.


Viscosity is well-known as the single most important attribute of any given lubricant. For motor oils, the viscosity selected must enable the lubricant to remain effectively pumpable at the absolute lowest start-up temperature that the vehicle will ever experience, while still having the capability to protect components at the temperatures experienced while in active service.

As a rule, diesel motor oil tends to have higher viscosity levels. If a driver were to put this high viscosity oil into a passenger car’s petrol engine, multiple problems are likely to occur. The first problem will be heat generation arising from internal fluid friction. Excessive heat impacts the life of the motor oil negatively. For every 10 degrees centigrade that the temperature of the motor oil is increased, the life of the oil is cut in half.

A further issue with using higher viscosity oil is that it has low temperature pumpability. In cold starts, the motor oil can be exceptionally thick and difficult for the vehicle’s oil pump to deliver to important engine components including the lifter valley. The impact of a lack of oil during start-up is premature engine wear, as the motor components will make contact without adequate lubrication, until the motor temperature begins to increase.

Additives per volume

Diesel motor oil has a greater number of additives per volume. Out of these additives, the most prevalent inclusion is over base detergent additives. These built-for-purpose additives have multiple functions; the main ones are to neutralise acids present and to clean the oil within the sump. Because of their fuel, diesel engines generate a far greater amount of soot and other combustion by-products. Via blow-by, these pollutants find their way into the engine’s crankcase, forcing the motor oil to deal with them. When such and extra additive load is added to a petrol-powered engine in a passenger car, the effects are disastrous and devastating to performance. The powerful detergent additive will work as formulated and will attempt to clean out the cylinder walls. Unfortunately, this can have a harmful effect on the seal present between the liner and rings, resulting in a loss of compression as well as efficiency.

To avoid any harm to your vehicle’s engine, only use the correct type of lubricant designed for its system.

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