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Oil Oxidation and Degradation: Causes, Effects, and Solutions for Diesel Engines

There are a number of things that can cause oil degradation, with one of the most prominent being oil oxidation.

Problems with your engine oil can cause no end of issues with your engine. Sludge, sediment, and deposits can all cause serious damage to your engine.

Fortunately, for every problem, there is a solution. We’re guiding you through everything you need to know about engine oil degradation and how to solve it. 

What Is Oil Oxidation?

As we mentioned above, oil oxidation is the most common cause of oil degradation.

Oxidation occurs when oxygen is added to a chemical substance, in this case, engine oil. As we all know, oxygen is pretty common what with us needing it to survive and all. Oxygen makes up around 20% of the atmosphere, so of course, it will get into your engine oil. 

Oxidation doesn’t just happen to oil. If you’ve ever heard your brake pads grinding after you’ve not driven for a few days in wet weather, that’s the rust on the metal. This is just another form of oxidation.

In the case of oil oxidation, as the oil reacts with the oxygen, oil molecules lose electrons. This is a very scientific explanation, but what it really means is the consistency of the oil changes. These changes can spell out problems for your engine.

Oil Oxidation Effects

This consistency change can result in the formation of sludge and deposits of oxidized oil. These reduce engine efficiency as well as increase the chances of engine failure. 

Engine efficiency is reduced in a number of ways. It can increase the engine oil viscosity. It can also deplete additives within the oil, reducing its effectiveness. These changes cause your oil to degrade faster overall.

Oil oxidation isn’t the only cause of oil degradation though. There are a few other common causes to be aware of. 

Thermal Breakdown

Temperature also has a huge effect on engine oil. This is because as well as separating the moving parts of your engine, oil also needs to dissipate heat for your engine to work.

So if your engine is running too hot and your oil is being heated too high, this can cause degradation. In fact, for around every 18°F increase in temperature, the life of your oil is cut in half.


Contaminants can also cause your engine oil to degrade faster. By contaminants, we mean things like dirt or water. Dirt with metal particles, in particular, can be a catalyst for oil degradation. 


Micro-dieseling happens when an air bubble within oil moves from a low-pressure point in the system to a high-pressure point. This air bubble temperature rapidly increases, which in turn increases the temperature of the surrounding oil molecules. This instantly oxidizes them.

It’s also known as a pressure-induced thermal breakdown. It’s most common in hydraulic systems. 

Avoiding Oil Degradation

Oil degradation is a natural process. All engine oil will eventually oxidize to some degree. So there’s no way to avoid this process completely.

This said, there are some things we can do to slow the process and keep our engines in better condition

Air Control

Preventing excess air from entering your lubrication system is one way of doing this. Air will enter your engine lubricant through loose connections. So ensure your fittings are tight in lubrication systems and keep bearing seals in good condition. 

Heat Control

Check which viscosity grade you should be using in your lubrication system. You can find this in your manual or by contacting your manufacturer.

If you’re using a low viscosity grade, your system will be allowing metal-to-metal contact. The friction generated will produce more heat. High viscosity can cause similar issues, so it’s important to use the correct grade. 

Regular Oil Changes

One of the most obvious ways to prevent oil degradation is with regular oil changes with high-quality oils and additives. 

Many synthetic oils now have oxidation-inhibitor additives. This helps prevent oxidation and gives you a more stable engine oil. Synthetic oils also have the benefit of being contaminant free. 

Signs You Need an Oil Change

If you’re a stickler for following the service schedule of your vehicle, you’ll already know how often you should be changing your motor oil. But not everyone is quite so on the ball, so there are some common indicators your car is overdue for an oil change

If you can hear unusual noises coming from the engine, this may be a sign your oil isn’t up to scratch. Knocking from the engine is usually metal components brushing together. Oil is supposed to prevent this and keep your engine quiet.

In the most severe cases, if you’re hearing loud rumbling sounds, this is a sign your engine is experiencing a lot of friction from a lack of lubrication.

Another sign is if you can smell oil inside your car. This can signify an oil leak. If you can smell gas, it’s a sign the vehicle may be overheating due to a lack of oil. 

Check your exhaust smoke. You’ll always have some level of translucent vapor from the tailpipe. If this changes to thicker or darker smoke, this is a sign of a problem with your engine oil. 

You can also check the color of the oil. Clean oil should be slightly translucent and amber. Older oil becomes darker and may have particles in it.

You can check the color the same way you check the volume of your engine oil. Take out the dipstick and wipe it, then return it to the oil tank. Then take it out a second time; if you can’t see the dipstick through the oil, then your oil is too dark and old. 

The most obvious sign you need an oil change is if your oil change light or check engine light comes on the dashboard. Do not just ignore these lights, they’re there to warn you to stop and take action to prevent damage to your vehicle.

Diesel Engine Repair

Oil oxidation is a problem for every vehicle owner.

If you don’t know how or don’t have the time to change your oil, or if you’ve left it too long and damaged your engine, we can help. We’ve got years of experience, specializing in diesel engine problems.

Contact us today to get booked in.