Choosing the right motor oil for your vehicle is one of the most critical decisions you can make for engine health and performance. The debate between synthetic and conventional oil has evolved significantly as modern engines demand more from their lubricants. Understanding the fundamental differences between these two oil types can help you maximize your vehicle’s performance, extend its lifespan, and potentially save money over time.
Understanding Motor Oil: The Foundation of Engine Protection
Motor oil serves as the lifeblood of your engine, performing multiple essential functions beyond simple lubrication. It reduces friction between moving parts, cleans internal components, cools the engine, protects against corrosion, and seals gaps between pistons and cylinder walls. Engine oil is crucial for keeping your engine running smoothly, and choosing the right oil can improve engine performance, extend its life, and even save you money over time.
What Is Conventional Oil?
The base oils used to make conventional motor oil are distilled from crude oil, which contains hundreds of different hydrocarbons. After extraction from underground reservoirs, crude oil undergoes a refining process that separates it into various fractions based on boiling points. Modern separation involves piping crude oil through hot furnaces, with the resulting liquids and vapors discharged into distillation units where they separate into petroleum components according to their boiling points.
Base oils distilled from crude contain several different molecules that are detrimental to lubricating a vehicle’s engine, and as a result, oil performance suffers. Conventional oil, which has a tendency to retain impurities collected within the engine, breaks down quicker, becoming less effective as a lubricant. These impurities and irregular molecular structures limit conventional oil’s performance under extreme conditions.
The Science Behind Synthetic Oil
Synthetic base oils aren’t distilled; instead they’re chemically synthesized, where refiners disassemble crude oil molecules to their fundamental components using various chemical reactions, then build synthetic base oils from the ground up using only uniform molecules. During synthetic base oil production, various methods such as chemical synthesis or hydrocracking are employed to create a pure and consistent base oil, ensuring excellent thermal stability, viscosity control, and resistance to oxidation.
What results is a pure base oil that doesn’t contain the mishmash of irregular molecules found in conventional base oils distilled from crude oil. These processes remove impurities from the crude oil and enable individual molecules to be tailored to the demands of modern engines, providing higher levels of performance and protection than conventional oils.
Poly-alpha-olefin (PAO) is a non-polar polymer made by polymerizing an alpha-olefin, designated as API Group IV and is a 100% synthetic chemical compound used as a base stock in the production of most synthetic lubricants. However, the term “synthetic” has become somewhat controversial in the industry. Several petrochemical companies have developed processes involving catalytic conversion of crude oil base stock under high pressures and temperatures in the presence of hydrogen to form very high-quality mineral lubricants known as API Group III, which are so highly refined that their properties almost match Group IV synthetics, and can now legally be called synthetic from a marketing standpoint.
Performance Advantages: How Synthetic Oil Outperforms Conventional
AAA found that, on average, synthetic oils outperformed conventional oils by 47 percent in conducted tests that evaluated several important physical, chemical and performance properties including shear stability, deposit formation, volatility, cold-temperature pumpability, oxidation resistance, and oxidation-induced rheological changes.
Superior Temperature Performance
Synthetics have a naturally higher viscosity index, meaning the viscosity changes less as temperature changes during normal engine startup and operating conditions. Synthetic oil also performs better in extreme temperatures, hot and cold. This stability is crucial because viscosity is an important property of lubricants that produces the film thickness or clearance between metal surfaces that slide or rotate against each other, and without this film thickness, excessive friction and wear would occur.
At extremely low temperatures, it is more possible for mineral oils compared to synthetics to become so thick that the oil is unpumpable or is unable to circulate effectively within the engine, which can cause lubricant starvation conditions and engine failure. Synthetics’ improved flow during cold starts also improves fuel economy, since they don’t contain waxes which are inherent to conventional oils, they don’t thicken as much when your vehicle sits overnight.
Enhanced Engine Protection and Longevity
Automakers are building cars with smaller engines for greater fuel efficiency and turbochargers to boost power, with turbocharged engines running even hotter than non-turbo engines where oil can exceed 400° Fahrenheit and the shaft inside a turbocharger can spin upwards of 200,000 revolutions per minute. Conventional and synthetic blend oils can break down faster under extreme turbo conditions and leave deposits on turbocharger components which can lead to failure, while full synthetic oils can protect these components much better.
When exposed to certain conditions, conventional mineral oils are usually more prone to chemical degradation compared to synthetics, and exposure to combustion byproducts, fuel contamination, water contamination, metal particles, acids, and extreme heat commonly occur in engines, where oil degradation can cause sludge, varnish or deposits, corrosion, viscosity change and impaired engine performance.
The advantages of using synthetic motor oils include better low-and high-temperature viscosity performance at service temperature extremes, better Viscosity Index, and chemical and shear stability, which helps in decreasing the loss due to evaporation and is resistant to oxidation, thermal breakdown, oil sludge problems and provides extended drain intervals.
Oil Change Intervals: Time and Money Savings
One of the most significant practical differences between synthetic and conventional oil lies in how frequently you need to change it. Unlike conventional oil, which needs changing every 3,000 to 5,000 miles, synthetic oil can last 7,500 to 20,000 miles, saving time and money. With synthetic oil, automakers usually recommend changing the oil every 5,000 to 7,500 miles, and some models are designed to go 10,000 to even 15,000 miles between oil changes.
Most modern vehicles have change intervals in the 7500-to-10,000-mile range, with some manufacturers even pushing these intervals to 12,000 or 16,000 miles. However, it’s essential to follow your vehicle manufacturer’s recommendations. Every oil decomposes, absorbs deposits, and loses its protective properties after a period of time, but synthetic oil degrades more slowly than conventional oil.
Time-based oil changes are scheduled by the calendar, usually every 6-12 months, while mileage-based oil changes are triggered by how far the vehicle has been driven, and both methods exist because oil breaks down in different ways. Driving adds mileage-related stress like heat, friction, and engine load, but oil can still degrade even when a vehicle isn’t driven much, as over time it can collect moisture, lose additives, or oxidize just by sitting.
Fuel Economy Benefits: Does Synthetic Oil Save Gas?
The relationship between synthetic oil and fuel economy is well-documented, though the improvements are modest. Synthetic oil allows for lower viscosity without compromising wear protection and better cold-flow, both of which improve fuel economy. The lower the oil’s viscosity – defined as its resistance to flow – the more readily it flows, and lower-viscosity oils are easier to pump through the engine and offer reduced resistance to moving engine parts, increasing fuel economy.
Motortrend did a study using a specific brand of synthetic oil, calculating how a small increase in fuel economy impacted costs over the miles, and they found that a vehicle with an average fuel economy of 20 mpg increased efficiency by 2 mpg, making the fuel cost savings about $150 over 12,000 miles. According to various studies and tests, synthetic oil can improve fuel economy by up to 5% compared to conventional oil, and while this may not seem like a lot, over time it can add up to significant savings on gas and maintenance costs.
Both oils lubricate the engine, but synthetic oil has lower viscosity, meaning it flows more easily inside the engine, reducing friction between moving parts, and this better flow means the engine doesn’t need to exert extra effort to turn parts. Synthetic engine oils are specially designed to reduce friction among moving engine parts, and their molecular structure ensures smoother surfaces and less energy expenditure on mechanical resistance, with the minor friction not only providing for high performance but also leading to less heat generation and avoiding extra stress, meaning the engine requires less fuel at the same rate.
Cost Analysis: Initial Investment vs. Long-Term Value
The upfront cost difference between synthetic and conventional oil is undeniable. While synthetic oils offer superior performance, they are substantially more expensive per volume than conventional motor oils. The high cost of quality base oil, expensive refining processes and pricey additives make synthetic oil more expensive than conventional oil.
However, the total cost of ownership tells a different story. When you factor in extended oil change intervals, better engine protection, potential fuel savings, and reduced engine wear, synthetic oil often proves more economical over time. While synthetic oil typically costs more up front, it offers longer service intervals and better long-term protection — which can lower overall maintenance costs.
Synthetic oil is generally worth the extra cost for most vehicles because synthetic oils are higher performing and more stable than conventional oils, created from purified base oils and special additives to resist breakdown better under high heat and stressful driving conditions, with the tighter molecular structure making them less prone to thickening in cold temperatures, and this exceptional stability allows synthetics to better retain viscosity, lubricating strength and protect engines for longer drain intervals.
Modern Engine Requirements and Synthetic Oil
Synthetic oil used to be something only performance cars needed, but now it’s the manufacturer-recommended engine lubricant in many mainstream vehicles, such as the Subaru Forester and the Toyota Camry. Modern internal combustion engines have tighter spaces between fast-moving metal parts to help make them more efficient, meaning these engines usually need lower-viscosity oil that flows more easily to form a cushion of lubrication between two parts, and at lower viscosity, synthetic oil holds up better in high temperatures.
Synthetic oil is quickly becoming the new normal, with nearly 70 percent of new cars in the 2019 model year getting either fully synthetic or a synthetic blend oil. Although synthetic oil was once largely found in high-performance vehicles, it’s being used in more mainstream engines so that automotive manufacturers can achieve improved fuel efficiency.
Environmental Considerations
The environmental impact of your oil choice extends beyond the immediate performance benefits. Synthetic oil is resistant to oxidation, thermal breakdown, oil sludge problems and provides extended drain intervals, with the environmental benefit of less used oil waste generated. Fewer oil changes mean less waste oil that requires disposal or recycling, reducing the environmental footprint of vehicle maintenance.
Additionally, the improved fuel economy associated with synthetic oil translates to reduced carbon emissions over the life of the vehicle. While the manufacturing process for synthetic oil is more energy-intensive than conventional oil refining, the extended service life and performance benefits can offset this initial environmental cost.
Choosing the Right Oil for Your Vehicle
Normally, you should use the oil your owner’s manual recommends. There’s no need to upgrade to synthetic unless your car falls under the ‘severe usage’ category, which includes taking short car trips that don’t allow the engine to warm up completely, not putting many miles on your car annually, using your car for towing, or living in a place with extreme hot or cold temperatures.
AAA recommends switching to synthetic oil especially for turbocharged engines, and for vehicles driven in “severe” conditions like hot summers, cold winters, towing, and stop-and-go driving. Manufacturers have a special set of recommended synthetic oil-change intervals for vehicles driven in severe conditions, including in extreme heat, extreme cold, dusty environments, or if the vehicle is used for towing.
Switching from Conventional to Synthetic
Switching from conventional motor oil to full synthetic motor oil is perfectly acceptable, as full synthetic motor oils are compatible with conventional oils, oils for high mileage vehicles, synthetic blends and other full synthetic oils, and engine flushing is not required. You can freely switch between synthetic oil and conventional oil as long as you always use oil that meets or exceeds the standard your vehicle’s manufacturer recommends, and it’s OK to mix synthetic oil with conventional oil, as long as each oil meets the specifications in your owner’s manual.
However, there are some considerations for high-mileage engines. If your vehicle has a high mileage engine with more than 75,000 miles that has never used synthetic motor oil, or one that has been poorly maintained with infrequent oil changes, it’s recommended to ease into extended oil change intervals to allow the oil to clean up the engine.
Understanding Synthetic Blends
For drivers seeking a middle ground, synthetic blend oils offer a compromise between conventional and full synthetic options. The combination of conventional and synthetic oils produces synthetic blend oil, which works similarly to conventional oils, but the addition of synthetics gives better protection to your vehicle’s engine. The benefit of synthetic blends is largely a cost-vs-benefit decision, as synthetic blends are cheaper than full synthetic motor oil but higher performance than conventional oil.
However, it’s important to note that there isn’t a set formula for synthetic blend oils, as one may contain Group I or II base oils with a Group III, IV, or V synthetic, and some synthetic blends may contain only 5% or 10% synthetic while another brand might have 50% synthetic in its blend.
Special Considerations for Different Driving Conditions
Your driving habits and environment significantly impact which oil type is best for your vehicle. Not all miles are equal, as city driving, stop-and-go traffic, frequent short trips, towing, and extreme temperatures all put more stress on engine oil, and if a vehicle sees heavy use or operates in tough conditions, synthetic oil can wear out faster, even if you’re under the recommended mileage.
Routinely driving your car without getting the engine up to operating temperature also qualifies as severe driving, and your owner’s manual likely includes recommended intervals for changing your oil when using your car in such conditions, but if it doesn’t and you want to play things safe, then you may want to change the engine oil as often as every 5000 miles or six months.
The Bottom Line: Making an Informed Decision
The choice between synthetic and conventional oil ultimately depends on your specific vehicle, driving conditions, and budget. Overall, full synthetic oils provide better engine performance and protection than conventional and synthetic blend motor oils. The 47% performance advantage demonstrated in industry testing, combined with extended oil change intervals, better fuel economy, and superior engine protection, makes synthetic oil the superior choice for most modern vehicles.
For vehicles operating under normal conditions with older engine designs, conventional oil may still provide adequate protection at a lower initial cost. However, for turbocharged engines, vehicles driven in extreme temperatures, high-performance applications, or severe driving conditions, synthetic oil is not just recommended—it’s often essential for maintaining engine health and maximizing longevity.
As engine technology continues to advance and manufacturers increasingly specify synthetic oil in new vehicles, the transition to synthetic lubricants represents the future of engine care. While the upfront cost may be higher, the long-term benefits in performance, protection, and peace of mind make synthetic oil a smart investment for most drivers.
Additional Resources
For more information on vehicle maintenance and performance optimization, explore authoritative resources from organizations like the American Petroleum Institute, which sets industry standards for motor oils, and SAE International, which establishes viscosity grades and performance specifications. The American Automobile Association (AAA) also provides valuable consumer research on motor oil performance and recommendations. For specific guidance on your vehicle, always consult your owner’s manual and consider speaking with a certified mechanic who understands your driving conditions and vehicle requirements.