Choosing the Right Viscosity for Your Car, Light Truck or SUV

Not so long ago, drivers didn’t have to think much about choosing the right engine oil viscosity for their cars – because there weren’t that many to choose from. But that has changed dramatically. The variety of makes and models of vehicles on the road has multiplied over the last few decades. In 2024, half of all cars purchased in the U.S. were imports, and more Americans drive foreign cars than domestic*. European and Asian models typically have different lubricant specifications from U.S. cars.

Moreover, people are keeping their cars longer – 12 years on average, according to recent studies, which means there is a wider variety of vehicle ages on the road with different lubrication needs. On top of all that, engine technology keeps evolving with each new model year, as manufacturers work to optimize fuel efficiency and reduce carbon emissions. Lubrication technology has to evolve to keep up.

Given all this change, it’s no surprise that the range of viscosities on the market today has expanded widely to meet the varied needs of today’s vehicles. Relying on “what your dad used” is no longer the way to choose the right viscosity. And fewer people are leaving it up to their auto service shop to figure out. A recent MarketWatch survey found that a surprising number of people – including around half of Gen Z and Millennial drivers – are taking responsibility for their own oil changes. That means more drivers are heading into auto supply stores or searching the internet for motor oil – and facing a baffling array of choices in formulations and viscosities.

So why is choosing the right viscosity important? Viscosity is a major factor in optimizing engine protection, fuel economy, and ultimately, your vehicle’s performance. Choosing the right viscosity will ensure you get the performance benefits that your engine is designed to deliver. Conversely, the wrong viscosity may impede performance or even lead to damage if the engine is not properly protected. 

Viscosity and Engine Protection

The main purpose of an engine oil is to protect the engine from friction and wear by forming a film between moving metal parts. Simply stated, viscosity is a measure of a fluid’s resistance to flow, or how easily it will flow to coat the parts it’s meant to protect. A lower-viscosity, or “thinner” oil, flows more readily into the engine components, while a higher-viscosity, or “thicker” oil, requires more effort to pump through the engine.

Of course, the internal engine temperature as well as the ambient outdoor temperature can alter the relative thickness of an oil. Think about honey – at room temperature, it can be thick and harder to pour, but once heated it becomes thinner and pours more easily. The oil viscosity numbers on the label – for example, 5W-20 – indicate how the oil will flow along a range of temperatures. 5W (the W stands for “winter”) means the oil will be sufficiently fluid for cold-weather starts, while the other number, 20, tells you the oil will produce a thicker film to provide the necessary protection when the engine is hot.

The Trend Toward Lower Viscosities

There is a common misconception – again harkening back to earlier times – that a thicker oil provides better protection than a thinner one. However, lubrication technology has evolved along with engine designs, and there is a clear trend toward thinner viscosities as automakers build more fuel-efficient engines. Modern engines are getting smaller and more compact, with tighter tolerances between moving parts, which calls for thinner, more fluid oils. Today’s oil formulation and additive technologies make it possible to deliver lower-viscosity oils that don’t compromise on protection.

Furthermore, a lower-viscosity oil requires less effort on the part of the engine to pump. That is why manufacturers are specifying lower-viscosity oils – they actually contribute directly to fuel economy by lightening the engine’s workload, which reduces fuel consumption.

Another trend to be aware of is the wider adoption of full synthetic oils and “synblends” that combine a synthetic and conventional base oil stock. There is no difference in viscosity between full synthetics, synblends and conventional oils. However, synthetics may deliver such benefits as stability in high temperatures, better oxidation resistance, and longer oil life. They cost more than conventional oils, but the cost will likely be offset by improved fuel economy and longer mileage between oil changes.

Check to see what viscosities the manufacturer recommends for your vehicle make and model. If you don’t have an owner’s manual handy, that information is usually available online. Keep in mind that the viscosity requirements may change as your car ages. Knowing and choosing the right viscosity will help you get the optimal performance over the life of your car.

*Made in America? These States Have the Most Domestic and Foreign Cars in 2025 | Insurify