Diesel Oil Change 101

What is Engine Oil?

Oil is an essential fluid that protects engine components from direct metal-on-metal contact, heat, and vibration. It coats parts with a protective layer that eases the movement of parts. Parts need a certain level of contact to transfer power. However, excess contact will mean forces such as friction and heat come into play.

Engine oil ensures that parts avoid direct contact, ensuring they don’t grind each other away. Engine oil performs three primary functions to keep your engine healthy and efficient:

  • Lubricate parts, easing movement.
  • Manages heat by reducing the amount of friction between parts.
  • Reduces wear and tear on your engine preventing metal-on-metal contact.

What is Diesel Engine Oil?

Diesel engine oil performs the exact functions of gasoline engine oils except at extreme levels. To understand the distinction and necessity, we first must understand the difference between gasoline and diesel engines.

The significant difference between these engine types is the explosion process, particularly in the combustion stroke. The explosion process is as follows:

Intake Stroke

During the intake stroke, the inlet valve opens, taking air in and pressing the piston downward. In gasoline engines, the air mixes with fuel. The same process happens in diesel engines except the air doesn't mix with fuel.

Compression Stroke

During the compression stroke, the piston moves upward, compressing the air. In gasoline engines, the compression stroke compresses the fuel and air mixture. In contrast, the compression process in diesel engines also heats the air up to 1004 degrees Fahrenheit.

Combustion Stroke

The combustion stroke is the process wherein heat is converted into energy, thus producing the force that enables the engine to power the car. In a gasoline engine, the spark plug ignites the fuel-and-air mixture. The explosion pushes the piston downward.

In a diesel engine, fuel is injected once the air is compressed to a high enough temperature. The heated air from the compression stroke causes the fuel to combust immediately, creating greater pressure and power than a gasoline engine.

Exhaust Stroke

The exhaust stroke is the final phase in a four-stroke cycle. During this stroke, the inlet valve remains closed. The piston moves up, pushing the burned fumes out through the open exhaust valve. After the stroke, another four-stroke cycle begins.

The exhaust stroke is the same for both gasoline and diesel engines.

Gasoline and diesel engines work differently and are exposed to different levels of pressure and heat. The key takeaways from the combustion process are:

  • Gasoline engines mix air and fuel during the intake stroke. Diesel engines do not. The compression of air in diesel engines heats the air.
  • Diesel engines have higher compression ratios. This is because the fuel is injected while the compressed air temperature is extremely high, creating a greater explosion and higher pressure.
  • Diesel engines have higher compression ratios than gasoline engines. (8:1 to 12:1 for gasoline engines and 14:1 to 25:1 for diesel engines) Higher compressions ratios yield greater power with less fuel at the expense of more violent ignitions.

The more violent nature of diesel engine combustion means these engines go through significantly higher heat, friction, and wear levels. This higher strain warrants and justifies the use and existence of diesel engine oils.

The Difference Between Diesel and Gas Engine Oil

We now understand the necessity behind diesel engine oil. It's time to delve into the key differences that separate diesel engine oil from gasoline engine oil. Diesel engines endure more significant levels of strain. It stands to reason that diesel engine oils will need specific properties to protect diesel engines. Here's how they compare to gasoline engine oil.


Viscosity refers to a fluid's resistance to flow. Higher viscosity levels are more resistant to flow, while lower viscosity levels flow more easily. You can compare honey (higher viscosity) to water (lower viscosity) for imagery. Diesel engine oils have a higher viscosity and lower pumpability, allowing them to endure the extreme conditions of diesel engine combustion.

Gasoline engine oils don't need higher viscosity as gasoline engines don't go through extreme engine conditions.

Additive Levels

Additives are interesting in that they modify specific properties of engine oils. This mitigates the deterioration of engine oils and helps them function in various conditions. Some additives include:

  • Detergents to mitigate the formation of sludge and accumulation of dirt on interior surfaces.
  • Anti-foaming agents that prevent air bubbles from forming in the oil.
  • Friction modifiers to reduce friction through zinc platelets that slide over one another between surfaces.
  • Corrosion inhibitors to combat the formation of rust on interior surfaces.
  • Viscosity improvers that facilitate the pumpability (the ease by which the oil pump circulates the oil) of oil over different temperature ranges.
  • Anti-oxidants that mitigate the effects of oxygen exposure on engine oil.

Both diesel and gasoline engine oils contain these additives. However, diesel engine oils contain higher concentrations (often up to 25% of the formulation). Again, this stems from the fact that diesel engines go through extreme conditions during regular operation.

Replacement Intervals

Despite the greater strain-bearing properties of diesel engine oil, they still have shorter oil change intervals versus their gasoline counterparts. Generally, you should always follow the recommended oil change intervals indicated by your owner's manual. (sans special use cases and driving conditions, as shown in the next section).

Gasoline engine oils typically need an oil change once a year or every 7,500 miles. In contrast, diesel engines require an oil change every six months or about 3,000 miles.

How Often Should I Change My Diesel Oil?

Every responsible car owner understands the importance of changing the oil at the right frequency. While there's nothing mechanically wrong with changing your oil too frequently, doing so means you'd be replacing perfectly good oil prematurely. It's an unnecessary expense that goes against practicality.

As a general rule, you should follow the maintenance schedule outlined in your owner's manual. However, it's also important to note that no two cars will ever have the same maintenance schedule. There are far too many variables in driving habits and driving conditions that make each car's oil change a unique case. Some factors that influence diesel oil change intervals are:

Frequent Stop-and-Go Driving

Frequent stop-and-go driving will shorten your oil change intervals because you'll rarely reach high speeds. A diesel engine creates much more gunk and quickly collects dirt at low speeds. This is due to the frequent oil pressure increases and drops.

It also means engines work harder because of the sharp braking and acceleration needed to move in a traffic jam. There isn't any inertia to help the engine move along consistently.

Frequent Towing of Heavy Payloads

Your engine inherently works harder when your vehicle pulls something heavy. It's less about the towing and more about how hard your engine works. Moreover, towing entails that your engine reaches a higher rpm for extended periods during towing.

Regular Exposure to Extreme Temperatures

Extreme temperatures affect the way oil behaves. Extreme cold can impede oil flow as though it were of higher viscosity. This means your oil may not properly lubricate your engine because it cannot sufficiently coat your engine's parts.

Inversely, extreme heat causes oil to behave as though it were thinner, diminishing its protective properties. Light oil will flow freely but will not protect your engine's parts adequately.

Note: You need not change your diesel engine oil to accommodate the seasons' temperature changes. A high-quality diesel engine oil will remain reliable across all temperature ranges.

Diesel Engine Modifications

Highly modified engines will require more frequent oil changes and higher-grade oils. The more violent the combustion, the thicker your engine oil needs to be. The more horsepower your engine produces, the thicker your diesel engine oil will need to be. If you're driving a track car, you'll need oil to handle the extreme temperatures associated with higher performance.

Can I Use Diesel Oil in a Gasoline Engine?

Yes, you can use diesel oil in a gasoline engine, provided it carries the API "SP" rating. Otherwise, you should avoid doing so. While it may sound strange to put diesel engine oil in a gasoline engine, there are real use cases for doing so.

Some car owners may have different cars and want to use one oil for all vehicles. Others may have a gasoline engine-powered race car. You can only use diesel engine oil in a gasoline engine if it carries an "S" or "SP" rating.

What Happens in the Diesel Oil Change Process?

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