Ask AMSOIL: What Happens if I Use the Wrong Weight (Viscosity) of Oil?

Ask AMSOIL: What Happens if I Use the Wrong Weight (Viscosity) of Oil?

Using a motor oil whose viscosity is too low or too high for your engine can invite a few problems, such as reduced fuel economy, increased chemical breakdown and increased wear. To understand why, you should first understand viscosity.

What is viscosity?

Viscosity is defined as a liquid’s resistance to flow. The higher an oil’s viscosity, the less readily it flows from a container when poured. Think of honey and water. Honey has a higher viscosity and flows more slowly than water.

Your vehicle owner’s manual specifies the correct viscosity of oil to use in your engine. Sometimes, it’ll specify a range of viscosities from which you can choose depending on weather. For example, it may recommend 10W-40 in warm weather and 0W-40 in cold weather.

The first number (the “10W” in 10W-40, for example) describes the oil’s ability to flow at 40ºC (104ºF). It’s helpful to think of the “W” as standing for “winter.” The lower the oil’s “W” viscosity, the more readily it will flow when it’s cold.

The second number (the “40” in 10W-40) describes the oil’s ability to flow at 100ºC (212ºF), or normal operating temperature. Higher numbers indicate the oil remains thicker when the engine is hot, which translates into better wear protection. A 0W-40 and a 10W-40 are the same viscosity when the engine reaches operating temperature, but the 0W-40 will flow better in cold weather.

Too low can invite wear

Using an oil that’s too thin can compromise wear protection over time. The oil should adequately fill the clearances between engine parts to help prevent metal-to-metal contact.

The crankshaft journals, for example, ride on a thin layer of oil that forms between the journal and bearing surfaces. If the oil’s too thin to properly fill the space and remain in place, the two surfaces will contact and wear out.

Extreme stress and heat add to the challenge. The oil’s viscosity has a direct bearing on its ability to develop a lubricating film of sufficient thickness to keep parts from contacting. Since oil thins as it’s heated, the already-too-thin oil becomes even thinner under extreme heat, worsening the problem.

Oil that’s too thin can also lead to insufficient oil pressure to properly support variable valve timing systems in newer engines. Low oil pressure may also result in the lifters not staying in contact with the cam lobes, causing noise and increased wear.

Too thick reduces fuel economy

Since higher viscosity oils offer better load-carrying ability, hence wear protection, you’d think using the highest viscosity oil possible would make sense. Why not use 20W-50 in everything?

Thicker oil, for one, is tougher to pump throughout the engine, which reduces fuel economy. The oil also won’t reach vital components at startup as quickly as lower-viscosity oils, inviting wear.

In cold weather, oil that’s too thick can hinder your engine’s ability to start, straining your battery at best, and leaving you stranded at worst.

Overly thick oil doesn’t transfer heat from engine parts as readily as thinner oils. Increased viscosity also increases internal friction, which increases operating temperatures. Higher temperatures cause oil to oxidize faster, or chemically break down. This eventually leads to sludge that, if bad enough, can block oil passages and starve the engine of oil, leading to engine failure.

To ensure best protection for your engine and to alleviate potential warranty concerns, it’s best to use the viscosity of motor oil recommended in your owner’s manual. If you have questions or concerns, contact AMSOIL Technical Services (tech@amsoil.com

16Jul 2018
land speed record

AMSOIL Helps Team FTF Cycles Achieve 8 Land-Speed Records Jamie Jarvi|Aug 14, 2017 2:02 PM The competitive realm of racing spans the landscape, encompassing virtually every form of transportation. The need for speed has found its way into everything from lawnmowers to heavily modified cars and motorcycles, leading enthusiasts on an endless pursuit to generate […]

12Jun 2018

Dealer Aiming for the Top It’s said that successful AMSOIL Dealers are fiercely independent. The same is said of Texans. Dealers Joann and Robert Smythe of Rosharon, Texas easily fit that description. They launched their Dealership about 18 months ago and are already Premier Direct Jobbers. “My husband and I have been selfemployed for most […]

12Jun 2018
Shitty transmission fluid verses fresh

Transmission Pan Drop vs. Flush: Which is Better? John Baker|May 08, 2018 10:56 AM It depends on what you want to accomplish. But, first of all, check your owner’s manual to see if your vehicle manufacturer recommends one instead of the other. If you want to ensure removal of nearly all the old transmission fluid, […]

12Jun 2018
Boyd Coddington of “American Hot Rod” attempts to bring this rust heap back to life.

Entombed 50 Years, Miss Belvedere Still Turns Heads Ed Newman|Apr 30, 2018 8:32 AM Miss Belvedere finally finds a home After 10 years of de-rusting and restoration, Miss Belvedere finally found a suitor in the form of Historic Auto Attractions, in Roscoe, Ill. The Smithsonian rejected her. And, while other auto museums offered to display […]

12Jun 2018
Break in engine period

Do Older Vehicles Need High Zinc Motor Oil? John Baker|Aug 25, 2017 8:37 AM Thanks to Hugh Ashburn for asking this question on our Facebook page.  In case you can’t click on the facebook link here is the entire question: We are Amsoil Dealers in Lewisville TX. My interest is in Older engines, and rebuilt older […]

04Jun 2018

Does NAPA Sell AMSOIL? (Where to buy AMSOIL) Daisy Quaker|Jan 12, 2018 10:11 AM Looking to buy AMSOIL products at your local NAPA store? The good news is, you can find AMSOIL products proudly stocked on the shelves of some stores. However, our products aren’t available in all retail stores. Here are some reasons why, and […]