For the majority of motorists, selecting the appropriate engine oil is a straightforward process: simply adhere to the manufacturer’s recommended specifications. Regardless of the make and model, the vehicle’s engineers have already precisely determined the correct viscosity for that particular car. Whether it’s 5W30 or 10W-whatever, this vital information is readily available in the all-important owner’s manual.
However, you might not be the typical driver.
Beyond Standard Recommendations
If you’re unsure about the required oil weight for your car in its factory configuration, consulting the manual is your first step.
Those involved in racing, operating with added boost, or having significantly increased their engine’s redline may need to determine their own specific viscosity requirements. Regardless of a car’s speed in stock form, the owner’s manual will not specify an oil for a three-hour endurance race or a custom-built turbo setup. This is precisely when thorough research and testing become essential.
Understanding Oil Flow and Viscosity Measurement
Every motor oil, whether it’s the most advanced synthetic lubricant or a budget petroleum product designed to meet a price point, is identified by its viscosity. Oil viscosity, also known as its weight, refers to its thickness or thinness; a lower weight indicates a thinner oil.
An engine demands the correct oil viscosity to perform optimally: the lubricant must be thin enough to circulate efficiently through the engine during cold starts, yet robust enough to maintain its lubricating properties when the engine is hot. Since a straight-weight oil thins when heated and thickens when cooled, these two fundamental engine requirements can conflict with the basic nature of a single-viscosity lubricant.
This inherent conflict is precisely why multi-viscosity oils were developed. These products exhibit different viscosities at varying temperatures; for example, a 10W30 oil behaves like a 10-weight oil at 0 degrees Fahrenheit (-17.8°C) and like a 30-weight oil at 212 degrees Fahrenheit (100°C).
Viscosity is measured under laboratory conditions. An oil sample is brought to the specified temperatures and then allowed to flow through a standard-sized orifice. The viscosity rating is determined by the duration it takes for the oil to exit the hole. A rapid flow results in a low rating, while a slow flow yields a high rating.
Adhering to Manufacturer Guidelines
In the vast majority of cases, we strongly advise using the oil viscosity specified by the car or engine’s manufacturer. For instance, if a Mazda MX-5 requires 5W20 from the factory, that’s the weight to use. If there are concerns that this oil might not provide sufficient film strength for that particular car and its intended use, the solution is to choose a better quality oil within that recommended viscosity—not to change from the specified viscosity.
Most automotive companies have transitioned towards lower-viscosity oils in recent years. This shift isn’t primarily due to tighter internal engine clearances, but rather because of governmental penalties imposed on manufacturers who fail to meet their average CAFE (Corporate Average Fuel Economy) mileage standards. Moving from a 5W30 down to a 5W20 can enable an automotive fleet to achieve a marginally better fuel economy figure; while this small improvement might not translate into a significant real-world difference for individual consumers, it can save manufacturers substantial amounts in avoided fines. (It’s worth noting that many OEMs have also subtly tightened their engine clearances in addition to using lighter oils, but this has been a very minor adjustment.)
The best-selling oil weight in North America is 5W30—and deservedly so, as it represents an excellent viscosity for flowing through the intricate pathways of a modern engine. This is why this particular weight is recommended by a multitude of automakers.
However, many “old-school” engine tuners often insist on running thicker oils. “We sell far too much 20W50 these days,” comments Cameron Evans of Red Line Oil. “It’s slowing down as people get clued in, but generally, it is far too heavy for almost every passenger car, and even many race cars.”
Many late-model cars come from the factory equipped with an oil cooler or are subsequently fitted with an aftermarket cooler. Track day sessions tend to be relatively short, further diminishing the necessity for truly heavy oils. A heavy oil can also leave an engine vulnerable when cold, Evans continues. High viscosities can also trap a significant amount of heat within the oil itself, potentially exacerbating an already problematic situation.
Charting Your Own Course: Modified Vehicles
Despite the usual recommendation to follow the owner’s manual, that helpful guide becomes less relevant when the factory setup has been fundamentally altered. Adding boost, significantly raising the redline, and engaging in extended track sessions can place immense additional strain on the oil.
In these specific scenarios, your oil temperature gauge will quickly become your most valuable tool: oil temperature serves as an excellent indicator for selecting the appropriate oil viscosity. Indeed, you will struggle to determine which viscosity to use effectively without the data provided by such a gauge.
Unfortunately, there’s a common tendency to immediately switch to a thicker oil without possessing this crucial data. This often occurs with mildly modified cars. An owner might install some basic bolt-ons—such as an intake, exhaust, ECU tuning, and so on—and then opt for thicker oil, assuming the engine is pushing the limits of what the factory 5W30 oil can handle.
As a result of using that heavier oil, the engine’s oil temperatures might never exceed 200 degrees Fahrenheit (93.3°C) while the car is on track. Consequently, the oil never reaches its optimal operating temperature to perform its job effectively and can actually hinder engine performance. Cold oil also accumulates contaminants more rapidly than warm oil, leading to shorter service intervals.
When experimenting with different oil weights in an engine, a high-quality oil temperature gauge is an indispensable tuning tool. The target temperature for oil is 212°F (100°C)—if the oil is excessively thick, it can actually keep the temperature too low, meaning the oil won’t provide adequate protection.
While the 212°F (100°C) mark is an excellent target, it’s not always realistic. So, if that same owner refills their engine with the factory-recommended 5W30 oil and then observes temperatures above 212°F—perhaps around 230°F (110°C) or even 240°F (115.6°C)—there’s still no need for immediate alarm. This is where a high-quality synthetic oil proves its worth, as it retains ample film strength at those elevated temperatures.
If oil temperatures consistently exceed 260°F (126.7°C), that serves as a clear indication to begin considering a different viscosity. The best synthetic oils are generally effective up to 300°F (148.9°C)—which is typically the point where engine seals can start to fail. Heavier oils also tend to carry more heat. For instance, if an engine is running hot with a 20W50 oil, switching to a stable 10W40 may very well lower the temperatures into a more desirable range.
It is also permissible to mix oil weights to fine-tune the viscosity for a specific application. For example, blending 5W20 and 5W30 might yield ideal oil temperatures; in other scenarios, a sump filled with a mixture of 5W30 and 10W30 could be the optimal solution.
Short of constantly disassembling an engine to inspect for wear, that critical oil temperature gauge will be your guide in zeroing in on the correct viscosity. While an old TV advertisement once boldly claimed that “motor oil is motor oil,” having the precisely right lubricant in your sump can significantly enhance both performance and engine longevity.
Race Oil vs. Street Oil
A car doesn’t automatically require pure racing oil just because it sees track use. While today’s racing oils often deliver increased lubricity, they typically lack the detergents commonly found in oils formulated for street use. Racing oils are also generally more expensive.
The desire for the absolute best possible oil makes true synthetic racing oils very tempting. However, before making that switch, objectively assess how your car is primarily used. Racing oils are designed for shorter change intervals, and consequently, they tend to omit some of the detergents present in oils formulated for street use.
While racing oils can effectively keep engine contaminants in suspension, they are not engineered to provide that level of protection for thousands of miles. However, forsaking some detergents does create more room in the chemical additive package for increased lubricity. Furthermore, it’s difficult to argue against a product that can potentially offer a slight boost in power, provided it aligns with your usage guidelines.
Dedicated road racing machines are clear candidates for these racing oils. Most road racers typically change their oil after each event, though some experts suggest that many modern race oils can endure two or three sprint race weekends before needing a change.
Most grassroots track day participants, however, would likely fare best with passenger car motor oils. Many of these vehicles cover substantial mileage between oil changes, and ultimate horsepower is not a prerequisite in these non-competitive events. The premium cost of today’s race oils—given that anti-wear agents are expensive additions—is another compelling reason for track day participants to continue using street oils.
Autocross Oil Considerations
Autocross and hillclimb competitors face unique challenges concerning oil heat, but their issue lies at the opposite end of the temperature spectrum. In these forms of competitive driving, insufficient heat becomes the problem, rather than excessive heat.
Unfortunately, many autocrossers and hillclimbers opt for oils that exacerbate this situation. A common scenario involves campaigning modified cars and assuming that their radically altered engines justify the use of a thicker oil. Where the specification for the stock engine might be 10W40 or even 5W30, they might choose 20W50.
They are overlooking a crucial piece of information. Most autocross and hillclimb cars arrive at the starting line with cold oil: not quite freezing temperatures, but certainly far from the target 212°F (100°C) mark. In these conditions, the 20W50 oil is likely to behave closer to its 50-weight rating, providing very little in the way of effective lubrication. The excessively thick oil is unable to penetrate deeply into the engine, which can lead to severe consequences such as spun bearings and thrown rods. A common, yet detrimental, follow-up action by too many enthusiasts in this situation is to switch to an even thicker oil, which only intensifies the problem.
Most autocross and hillclimb cars—even those that have been modified—will perform optimally with the oil viscosity recommended by the manufacturer. Before pushing the accelerator, ensure the oil reaches its proper operating temperature so it can effectively perform its vital functions. If a top-notch oil is in the sump, autocrossing alone should not cause undue engine wear.
A technically proficient driver equipped with an accurate oil temperature gauge can experiment with slightly lighter oils. These will reach optimal operating temperature more quickly than heavier oils, potentially offering a slight power advantage, yet still providing adequate engine protection under these unique driving conditions.