Diesel engines have been growing in popularity in recent years, especially for trucks, SUVs, and other vehicles that require a lot of power. With this increase in diesel engine use, many drivers wonder whether synthetic oil is better suited for diesel engines than conventional oil.
In this blog post, we’ll take an in-depth look at the differences between synthetic and conventional oil and discuss the potential benefits of using synthetic oil specifically in diesel engines. We’ll also provide a clear answer on whether synthetic is truly better for diesel applications.
| Variables | Conventional Oil | Synthetic Oil |
| Base Oil | Crude oil | Synthesized chemical compounds |
| Viscosity Stability | Degrades over time | Maintains viscosity in wide temperature ranges |
| Contaminant Tolerance | Lower | Higher |
| Oil Change Intervals | 3,000-5,000 miles typical | 7,500-15,000+ miles possible |
| Cost | Lower | Higher |
| Cold Flow Properties | Poor | Excellent |
| Oxidative Stability | Lower | Higher |
Table of Contents
What is Synthetic Oil?
Synthetic oil is a lubricant consisting of chemical compounds that are artificially made. Unlike conventional motor oils that are refined from crude oil, synthetic oils are custom-synthesized in a lab to produce oil molecules with enhanced qualities. The base oils in synthetic oil are typically derived from natural gas or other petrochemical feedstocks.
Synthetic motor oils were first developed in the 1940’s by German scientists, though they did not become commercially available until the 1970’s. Today, synthetics are used in many high-performance applications thanks to their superior properties compared to conventional oil.
Some key advantages of synthetic motor oils include
- Excellent viscosity stability even in extreme temperatures
- Resistance to oxidation and sludging
- Cleaner operation and reduced deposits
- Improved fuel economy
- Better cold weather flow
These benefits make synthetic oils desirable for many engine applications, but are they necessarily better for diesel engines? Let’s take a closer look.
What Makes Diesel Engines Different?
Diesel engines have some key differences compared to gasoline engines that affect lubricant performance:
- Higher compression ratios – Diesel engines typically operate at 14:1 to as high as 24:1 compression ratios, far higher than gasoline engines. This puts more pressure on the oil film.
- Higher combustion temperatures – Diesel fuel ignites at lower temperatures than gasoline, leading to higher combustion chamber temperatures reaching over 1000°F. This increases thermal stress on the oil.
- Contaminants from fuel combustion – Diesel fuel contains sulfur and other compounds that end up in the crankcase oil, reducing the oil’s lifespan.
- Turbochargers – Many diesel engines are turbocharged, requiring the oil to lubricate and cool hot, fast-spinning turbo components.
These factors mean the oil in a diesel engine has to withstand more punishment than the oil in a comparable gas engine. Next, let’s see how synthetic and conventional oils stack up in dealing with these diesel-specific demands.
Synthetic Oil vs. Conventional Oil in Diesels
Here is a detailed look at how synthetic and conventional oils compare when used specifically in diesel applications:
Viscosity Stability
An oil’s viscosity refers to its resistance to flow. Having stable viscosity is crucial for maintaining a protective oil film across a wide temperature range.
Conventional oils use base stocks that are prone to viscosity breakdown at high temperatures. As the oil gets thinner, the lubricating film on critical engine parts can be compromised.
In contrast, synthetics maintain their viscosity even at extremely high temperatures inside the cylinder. This ensures maximum wear protection despite the intense heat.
Winner: Synthetic
Oxidation Resistance
When oil oxidizes, it forms sludge, varnish, and deposits that clog engine parts. This problem is exacerbated in diesel engines by contaminants entering the crankcase from fuel combustion.
Synthetic base oils have inherently higher oxidative stability than mineral oils. Their uniform molecules are highly resistant to reacting and decomposing. This allows synthetics to last longer before forming harmful byproducts.
Winner: Synthetic
Cold Flow Properties
Diesel fuels often contain paraffin waxes that can gel in cold temperatures and starve the engine of fuel. To combat this, diesel oils have “winter grade” variants tailored for low temperature operation.
The pour point – the lowest temperature an oil can flow – is much lower for synthetic than conventional oil. Synthetics also have improved cold flow so oil circulates faster at startup to minimize wear.
Winner: Synthetic
Engine Wear Protection
The advanced additive packages in both synthetic and conventional diesel oils provide ample protection against scuffing, scoring, and piston and ring wear. Both offer sufficient anti-wear performance for diesel engines.
Winner: Draw
Soot Control
Diesel engines generate significant soot contamination from the incomplete combustion of fuel. This soot can quickly thicken the oil if not properly dispersed.
Dispersant additives work to suspend soot in tiny particles so it can’t accumulate on surfaces. Synthetics frequently use superior dispersants that combat sooting better to enable extended drains.
Winner: Synthetic
Contaminant Tolerance
Fuel dilution of the oil is a concern with diesel engines that see mostly low load, low temperature duty cycles. This contamination reduces the oil’s lubricity.
The narrow molecular range of synthetics makes them inherently more resistant to fuel dilution compared to conventional oils. Their performance is not as drastically impacted by contamination.
Winner: Synthetic
Oil Drain Intervals
Diesel oils contain detergents to neutralize acids from fuel combustion products that would otherwise corrode metal surfaces. As these detergents deplete over time, shorter drain intervals are required.
The advanced formulations and contaminant tolerance of synthetics allow them to safely extend drain intervals up to 2-3 times longer than conventional oils, provided regular analysis is done.
Winner: Synthetic
Verdict: Is Synthetic Better for Diesels?
Based on the above comparisons, synthetic oil provides numerous advantages over conventional oil when used in diesel engines:
- Superior stability at high temperatures
- Enhanced oxidation and sludge resistance
- Better cold flow properties
- Extended drain capabilities
- Increased contaminant tolerance
While conventional diesel oils still offer adequate performance for many applications, synthetic oils are clearly better suited for handling the demanding conditions inside modern diesel engines. Their advanced chemical properties translate to better wear protection, cleaner operation, and increased engine longevity compared to mineral-based oils.
For these reasons, we recommend choosing a full synthetic diesel oil if you want the highest level of protection and performance from your engine. The upfront cost is more than worth it considering the extended drain intervals and reduced maintenance synthetic oils provide. Just be sure to follow the recommended drain schedules outlined in your owner’s manual.
With high-quality synthetic oil combined with regular oil analysis, you can confidently push your next oil change further and maximize your diesel’s potential.
Frequently Asked Questions
If you’re considering switching to synthetic oil in your diesel, there are likely some lingering questions. Here we address the 6 most frequently asked questions about using synthetic diesel engine oils:
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Is synthetic oil mandatory for modern diesel engines?
Synthetic oil is not required for diesel engines, though it is highly recommended for optimal protection, especially in newer, high-tech diesels. Conventional diesel oils still offer adequate performance if changed regularly.
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Can you mix synthetic and conventional oils?
It’s generally not recommended to mix synthetic and conventional oils since their chemistry differs. Stick with one oil type across drain intervals for best results.
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How often should I change synthetic diesel oil?
Follow the drain intervals specified in your owner’s manual, which are usually around 7,500-15,000 miles with synthetic oil. Used oil analysis helps determine optimal change frequency.
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Do all synthetic diesel oils perform the same?
Not all synthetics are equal in performance and quality. Opt for well-known brands that meet strict specifications like CK-4, CJ-4, or FA-4. Avoid cheap synthetic blends.
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Will switching to synthetic oil cause leaks?
Good quality synthetic oils will not cause seal leaks in an otherwise healthy engine. However, they may expose existing minor leaks by thinning sludge that was plugging gaps.
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Can synthetic oil extend the life of my diesel engine?
Synthetics provide enhanced protection against wear, deposits, and other damage, helping maximize engine longevity when used properly. But all engines have finite lifespans.
The Bottom Line
If you want to get the most protection and performance from your diesel engine, a high-quality full synthetic oil is the best way to go. Synthetics outperform conventional oils across multiple metrics critical for combating the demanding conditions inherent in diesel engines. With improved stability, contaminant tolerance, and cold flow, synthetics can help clean up your engine, extend drain intervals, and maximize the lifespan of your diesel. The next time your oil change comes around, make the switch to synthetic diesel oil. Your engine and wallet will thank you.
