Is there a single test to say how degraded a lubricating oil is?

An interesting question today from a customer regarding how oils degrade and I thought I would make an article to answer.


“Hi Learn Oil Analysis,

I have heard lots of conflicting information about what is the best way to test how degraded an oil is. Some say its measuring varnish potential, others measuring RULER and others RPVOT. Which one should I use?

Hope you can help 🙂



This is a great question as being a lab owner myself everyone is constantly trying to sell you their new way to test oil and all claim to be the silver bullet to highlight all issues. Some of these are great, but ultimately it can often be a combination or a matter of timing and what you want to achieve. So firstly lets cover what is a lubricant made up of?

What does a lubricant contain that we are looking to measure?

Lubricating oils are made up of two main constituents, namely the base oil and the additives. In terms of additives, we are interested in anti-oxidants which are usually phenols, amines or anti-wear based or a combination of all 3. For a lubricant to form sludges and varnishes which can cause problems in your machines both the anti-oxidant additives and the lubricant base oil natural oxidation-resistant properties need to be overwhelmed.

With additives, the more of these additives present the more anti-oxidant properties the oil will have, but some combination additives have a recycling effect to rejuvenate the anti-oxidant over time hence it is not a simple case of the more the better in every single instance as some combinations of additives may be better than the sum of their individual parts.

With base oil, taking petroleum-based oils which is the vast majority still of lubricants these are divided into different classes generally groups 1 to 4. The higher the group the more highly refined or in the case of group 4 the more highly synthesised the lubricant meaning the parts of the lubricant molecules that make them susceptible to oxidation have been largely removed such as double bonds and none carbon-hydrogen and carbon-carbon bonds. Without going too much into the chemistry it is easier to think for groups 1 to 3 the higher up the groups you go the more weak links you are taking out of the lubricant in the defences against oxidation, wherewith group 4 you just build the defences without weak links in the first place.


What is needed for a lubricant to degrade?

This is a very big topic and much research is done into this with millions invested to try to make lubricants last longer, but I will keep this brief for the purposes of the explanation below.

Lubricants degrade oxidatively through 3 sources namely oxygen, heat and a catalyst such as iron.

  • Oxygen – Oxygen is part of the air we breathe so reducing its presence in the machinery is exceptionally difficult even in sealed systems as the head of tanks contains air and so does lubricant storage etc.
  • Heat – For every 10’C we increase the temperature we nominally half the life of the oil. So if you have two machines with one running at 40’C and another at 60’C this means the one running at 60’C with all other things being equal will have only 1/4 the life of the other machine (half and then half again for each 10’C rise).
  • Catalysts – the machinery will often be made of the catalyst materials such as iron, hence this usually cannot be avoided, but high wear rates greatly increase the surface area of the fresh surface metal that has not been inactivated by anti-wear additives and will increase oxidation.
  • Water – acts as a medium for the oxidation reactions to take place and allow ions to be stabilised during their intermediary steps in the reaction. These also hinder and strip the lubricant of protective additives.

How does a lubricant degrade and how to test for it?

Please have a read of the diagram below whilst reading through these steps to assist in putting this into context.

Step 1 – Measure Anti-oxidant concentrations

Step one involves the anti-oxidant additives being spent. This can sometimes be seen with a simple colour change or darkening of the lubricant as many phenols are used as dyes and as they react they change colour. However, very mild changes in additive concentrations can cause huge changes in colour and you can have very similar colours with massively different additive concentrations. Colour can be tested using the ASTM colour scale. In addition to colour, measurement of individual additive concentrations can be used with popular methods including the RULER test which measures as a percentage the amount of anti-oxidant vs a new oil. The limitation of this method is that it assumes a few things:

    1. Customers don’t mix oils – In reality, this is not true as even with a complete drain of a lubricant about 10% remains purely as the surface tension of the old oil against the internal machinery surfaces and pipework.
    2. Oil Companies always keep the same formulation for a product – In reality, this is not true. Additives are constantly being tweaked so the reference oil from one year to the next is unlikely to be exactly the same. Equally, even for the same product there will be batch differences and different manufacturing plants will use different versions of a formulation. For instance, I have known two UK blending plants for the same oil major give two completely different additive packs for the same product because as long as the oil meets the approval of the OEM it doesn’t matter which additives the oil company chose to achieve it and they can call the products the same name. This is really common with worldwide formulations where products in e.g. Germany are different to the USA etc.

Hence comparing to a virgin oil that has not been mixed and may be a different or outdated formulation can give a false picture and I have seen RULER values of 130% before (I.e: used oil is better than the new oil). It also gives no ability to compare between products, which means lube oils with with very low anti-oxidant concentrations look as good as lubricants with far higher additive concentration products. I personally prefer the Oil Analysis Laboratories anti-oxidant ppm methods where the values are expressed in concentrations of ppm and can be compared between different oils.

Step 2 – Measure Oxidation

Measuring oxidation by infra-red analysis identifies the lubricant beginning to oxidise. This step may be a very short or very long stage depending on the base oil quality, with the higher quality base oils lasting for longer. Hence why simply changing when the anti-oxidants are spent can be a false economy especially when using highly refined or synthetic lubricants that may have plenty of life left in them. Oxidation is a good indicator of the base oil beginning to degrade but does have interferences with certain additives giving an artificially high result when e.g. ester additives are present.

Step 1 & 2 combination – Measure Oxidation Resistance

This is commonly performed by the RPVOT aka RBOT (Rotating Pressure Vessel Oxidation Test), which involves taking a sample of the oil in combination with a catalyst and pure oxygen under pressure to attempt to artificially and rapidly degrade a lubricant. The test is measured in minutes with a common cut of >500 minutes being a good pass result and <100 minutes being considered a poor result. The reason why this test is a truer reflection of lubricant degradation than antioxidant methods like the RULER test, is it is measuring both the anti-oxidant and base oil resistances together. Although the diagram shows additives spent first then the base oil, in reality it is both being degraded the entire time, but predominantly the antioxidant additives at the beginning and as this protection is removed it then includes the base oil. RPVOT is the gold standard test in the industry for many decades now but does increase the sample price to cover the high consumables such as the copper catalyst coils that are used with each test.

How does the RPVOT work in detail?

The Rotating Pressure Vessel Oxidation Test (RPVOT, aka RBOT) uses an oxygen pressured vessel to evaluate the oxidation stability of new and used oils with water and a copper catalyst coil at 140°C or 150°C depending on the variation of the method used. The oil is electrically heated in a dry pressurised bath with pure oxygen and the sample rotates axially at 100 RPM at a stable pressure of approximately 90 psi. Hence we have all the ideal conditions for oxidation with a copper catalyst, heat, water and pure oxygen. As the oil oxidation resistance of the base oil and additives is overcome the oxygen forms part of the lubricant molecules and hence is no longer exerting a pressure as a gas. This drop in pressure below a threshold as dictated by the method is considered the end of the test and the resistance is considered to have been overcome.

Step 3 – Acidity and varnish

At this stage, the oil may be beyond salvage as the oil begins to chemically break down forming free radicals that rapidly increase the rate of oxidation. Hence things can change very quickly at this part of the process and decisions may need to be made rapidly to prevent failures. One of the products is varnish which can block, coat and seize components and also insulates the system to prevent heat loss, which also increases the rate of degradation. This eventually leads to wear increases from the more acidic environment and poor lubricant properties of sludged oil, which is also a catalyst for degradation. Hence you can see how everything is feeding back into the process to generate further degradation at this stage and the end result is a failure. This can be e.g. a seized component, blocked filters, system overheating or other serious mechanical failures. Hence this is the stage you often want to avoid.

lubricating oil degradation and ways to test Is there a single test to say how degraded a lubricating oil is?


Which is the best test?

There is no “BEST” test in this regard for all scenarios and although many labs will have preferences of methods to use it depends on your individual situation and what you are trying to achieve. If you are in the middle of nowhere and have no access to a lab and don’t care how your oil compares to other oils then something like a remaining useful life or another portable voltammetry tester would be ideal as they are designed for field use and are very quick and simple to perform for non-lab people. If you want to get a top to bottom full health check assessment of your oil’s level of degradation then you may choose to go for instance, for an Oil Analysis Laboratories anti-oxidant test plus an RPVOT and a varnish potential. It all really depends on the criticality of your machinery and what you are trying to achieve. If you are unsure what you need why not use the contact us button at the bottom right of this screen and get in touch explaining a bit about your machinery so we can best advise you what tests would help identify the degradation of your oil. We will also be able to help you with detecting the wear and contamination of your oil which are also very important aspects to measure in any lube analysis programme.