What happens when you use thicker oil in your car

what happens when you use thicker oil in your car

It is best to use the recommended viscosity, but a slightly thinner or thicker oil can do no harm.

People often ask whether it is safe to use an oil of thicker or thinner viscosity than that recommended by the engine manufacturer. A common question is whether it is safe to use 10W-30 motor oil instead of, for example, 5W-30. Although it is best to use what is recommended in the owner's manual, inadvertently using a viscosity grade higher or lower than recommended is usually not harmful in the long run.

Two examples illustrate this point.

Brad bought a new car that needed 0W-20 oil, knowing the old adage that "higher viscosity oil equals better wear protection."

To give his new car the best possible protection, he ditched the original 0W-20 oil and installed 15W-50 racing oil.

Next up was John. To save money, he bought a 1998 Toyota Corolla that needed 5W-30 oil. But he had 10W-30 in the garage and used it for oil changes. No sense in wasting good oil.

Thicker oil (higher viscosity).

It is best to use the oil viscosity recommended in the owner's manual, but using slightly thinner or thicker oil (lighter or heavier viscosity) is usually not harmful in the long run.

Engines are designed to use a specific engine oil viscosity.

Today's modern engines are built to much tighter tolerances than their predecessors. For example, the clearance between the crankshaft journal and main bearing is tighter. This was done deliberately so that modern engines could use lower viscosity engine oils, such as 0W-20 or even 0W-16.

Lower viscosity oils improve fuel economy by reducing internal friction, as they flow more easily than higher viscosity oils. As fuel economy standards become more stringent, vehicle manufacturers are turning to lower viscosity fluids to meet the requirements.

Thicker oils may not flow fast enough.

In Brad's case, his 15W-50 racing oil may have been too thick and did not flow fast enough to fill the space between the crankshaft journal and main bearing when the engine was running.

The oil does not form a uniform lubrication film and therefore metal-to-metal contact and wear may occur. Your engine is specifically designed to use a lower viscosity oil, in this case 0 W-20. Because of its lower viscosity, it flows faster and fills the small spaces between parts, forming a long-lasting, uniform lubricating film.

In addition, the engine loses energy when pumping thicker oils, which reduces fuel consumption. Because thicker oils cannot transfer heat as well as thinner oils, operating temperatures increase, which can lead to accelerated chemical breakdown (known as "oxidation") and harmful deposits and sludge.

More information on oil viscosity can be found in this publication.

Some of the differences in oil viscosity are less obvious.

In John's case, using 10W-30 instead of 5W-30 would have caused fewer potential problems.

His older engine was not built to the same tolerances as Brad's engine. Also, once the engine reaches operating temperature, the viscosity of both oils is the same. You know this because the second number in the viscosity specification (e.g., "30") is the same for each oil. It describes the flow resistance of the oil at 100°C (212°F) or normal operating temperature.

However, using 10W-30 instead of 5W-30 can make cold starting more difficult.

It is worth thinking of "W" as "winter". The lower the "W" viscosity of the oil, the easier it will flow in cold conditions. In this case, 5W-30 flows more easily at start-up than 10W-30.

Some automakers even allow switching to lower viscosity oils over time.

SYNTHETIC VS. MEETING

You may know that synthetic oils are better than conventional oils, but you may not understand why. These differences start at the molecular level.

What if John had gone crazy and used 0W-16 in his 1998 Corolla instead of the recommended viscosity?

Just as using an oil with too high a viscosity can cause Brad's engine problems, using a thinner oil can have the same effect.

Oils that are too thin do not form a uniform lubricating film, which promotes metal-to-metal contact and leads to wear.

The rough situation

Extreme pressures and high temperatures are an additional challenge. Since oil thins when heated, oil that is already too thin becomes even thinner at high temperatures, which exacerbates the problem.

Oil that is too thin can also result in insufficient oil pressure for the vehicle's variable valve timing system (if equipped) to function properly. Low pressure can also cause the pushrods to fail to maintain contact with the cams, which can lead to noise and increased wear.

A low oil level can also result in insufficient oil pressure to allow the vehicle's variable valve timing system (if equipped) to function properly. Low pressure can also cause the pushrods to fail to maintain contact with the cams, which can lead to noise and increased wear.

A lesson on using thick or thin oil.

Using a viscosity one grade higher or lower than recommended for your engine is unlikely to cause permanent damage.

However, to alleviate engine protection and vehicle warranty, it is best to use the viscosity recommended in the owner's manual.