Why Do I Need To Change My Oil?

Regardless of its quality, every motor oil eventually loses its potency and must be changed to ensure peak engine protection. Let’s take a look at what happens to motor oil over time and why you periodically need to change oil.

Losing the base

Base oils are the backbone of the finished lubricant that ends up in your engine. Over time, they lose effectiveness due to the following factors:


The interaction between oxygen molecules and motor oil molecules naturally leads to chemical breakdown.

Just as oxygen causes a cut apple to brown or exposed metal to rust, it breaks down base oils and reduces motor oil’s effectiveness.

Oxidation can lead to increased oil viscosity, which negatively affects energy efficiency. It also causes the formation of harmful deposits and sludge.

High heat

Today’s engines run hotter than ever before, with temperatures up to 235°F (113ºC), and even higher if towing or hauling.

The rate of oxidation for oil doubles for every 18°F (10°C) increase in temperature.


Your vehicle is subjected to temperature swings, even when it is parked in the garage.

Those temperature swings cause condensation to form inside your engine, leading to water contamination.

Leaving a vehicle parked for extended periods or taking short trips that don’t allow the engine to fully warm up allow water to remain in the oil rather than evaporating and exiting through the tailpipe. Water can lead to formation of sludge…yet another reason why you must change oil.

Kinematic Viscosity-Oil vs Honey
Viscosity is a motor oil’s most important property. The lower the viscosity, the faster the oils flows, like water. Thicker oils flow more slowly, like honey.

Viscosity loss

A lubricant’s viscosity is its most important property.

Viscosity has a direct bearing on wear protection, and your engine is designed to operate best using a motor oil of a specific viscosity (e.g. 5W-30).

The intense pressure the oil bears as it’s squeezed between moving parts, like the piston ring/cylinder wall interface, can tear apart, or shear, its molecular structure, leading to viscosity loss.

Suddenly, the 5W-30 motor oil your engine was designed to use is now essentially a 5W-20 oil, and wear protection may be compromised. When this happens, it’s time to change your oil.

Fuel dilution

Fuel can wash past the piston rings and contaminate the motor oil, causing it to lose viscosity.

Frequent short trips that don’t allow the oil to reach normal operating temperature can be especially problematic because the fuel won’t volatilize and exit through the PCV system.

Excessive fuel dilution leads to sludge and varnish, requiring you to change oil more frequently.

Additives: designed to deplete

Additives are added to base oils to reduce destructive processes and enhance beneficial properties.

For example, antioxidant additives help slow the rate of oxidation. Detergency additives help prevent deposits and sludge while cleaning pre-existing deposits. Formulators add anti-wear additives to some lubricants to form a sacrificial barrier on metal components and help prevent wear.

Since they’re sacrificial in nature, additive depletion is one of the primary reasons motor oil loses its effectiveness and must be changed. 

While AMSOIL synthetic motor oil gives you the convenience of fitting oil changes into your schedule, it remains vital to install fresh oil at the appropriate time.  

Updated. Originally published Sept. 9, 2016.


  1. I had no clue that if you didn’t change your oil, it could lead to rusting inside of your tank. I’m borrowing my friend’s car for the week and he gave me some money to run some maintenance checks on it while I have it. It looks to me that a car oil change service will be really important. I wouldn’t want his car to rust or become inefficient due to unchanged oil so I’ll be sure to keep that in mind, thank you for telling me.

  2. How often would you recommend changing oil (currently NAPA High Mileage Synthetic Blend 5w-20) in a ’93 F350 with a 460 that gets driven less than 5,000 miles per year, but those miles are usually an hour or two on the highway then a couple hours on mountain roads, usually with a heavy load or towing. (I would use AMSOIL but it is far out of my budget right now.)

    1. Hi Jonathan,

      We don’t make recommendations for other oils. Follow the guidelines in your vehicle-owner’s manual or the recommendation on the oil-bottle label.

      Let me ask you this: AMSOIL products cost more compared to what? True, the initial retail price is sometimes higher than other oils, but you can recoup that money by going longer between oil changes. That’s not to mention the benefits of better wear protection, reduced oil consumption, maximum fuel economy, a cleaner engine and so on. This is especially important if you’re towing and hauling.

      Next time you change oil, consider stepping up to AMSOIL XL Synthetic Motor Oil, which is good for up to 12,000 miles or one year.



  3. Changing oil for your car is regularly. Maybe every 2-3 months so that you can have a healthy oil.

  4. I have a 2000 Volvo S80 and drives approx. 10,000 miles a year primarily in city traffic. I use 10w-30 Amsoil Signature Series motor oil rated at 25,000 miles. As I do not drive much do you recommend changing my oil before the 25,000 miles? If so, how frequently in months or miles should I change the oil?

    1. Hi Steve,

      Follow our recommendation for Signature Series. Due to the frequency with which you drive in city traffic, your driving conditions probably fall under the “severe” designation. That means you can safely use Signature Series for up to 15,000 miles/one year. Follow those guidelines for the best value.



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