4 to 5 stars – Understanding car engine oil analysis in 5 minutes

Normally I get questions submitted by email, but this one came from a review we had on Google business from Peter. Peter was sampling his own vehicle and his review said the only thing that was stopping him from giving us a 5 star review was he didn’t have a simple guide specific to oil analysis parameters on engine oils for cars.

There is a wealth of information on LearnOilAnalysis.com and Peter has actually already subscribed to the articles to go to his inbox so may indeed read this article, but I thought let’s exceed expectations and give the customer the one bonus thing they wanted. Now, for clarity, we do have some vehicle alarm limits that cover all road vehicles and also some specific guides on engine oil analysis, but never one to settle for 4 out of 5 I thought I would take up the mantle and address this question. So here we go…

So how is an oil report rated?

We have 4 rating scales of Normal (A), Caution (B), Serious (C) and critical (X). In this case, critical is the worst state and Normal is the best. Most people never read the normal reports as you often dont need to take any action, but if the sample is abnormal (B,C or X) then that means there is a potential issue and we will discuss this in the diagnosis at the top of each page. It is often in the format of commenting on what’s important from the lab data and then giving some advice on the back of this data.

Beneath the comments at the top, is for the uninitiated, the scary bit with of lots of tests and numbers. Dont worry though, we only comment on the ones you need to look at, so use the comments to focus your reading of the data. In most cases the sample will be satisfactory, but should it not be there will likely be a couple of potential failed tests. So let’s cover each of these in groups.

What are the units of the tests?

Most the tests will be in percent I.e. out of 100 or in ppm aka mg/kg meaning out of 1 million (parts per million).

You can interchange the units quite easily by moving the decimal 4 places.

1000 mg/kg = 0.1%

100mg/kg = 0.01%


Physical tests – Viscosity

The main physical test and actually the most important test is called viscosity or the thickness of the oil. The whole purpose of the lubricant is to separate the surfaces so you don’t get metal to metal contact.

As viscosity changes with temperature as oils get thinner when they get hotter you have a couple of standard temperatures agreed for testing and these are 40 and 100C.

Too thick could mean the oil is oxidising and old and sooted whilst too thin could indicate fuel dilution for instance. We will tell you what we think the reason is in your individual report.


Base number – also known as BN or TBN is the ant-acid tablet of your engine. This is put in by oil manufacturers to neutralise acids of combustion. It is a great indicator of remaining life when compared to the new oil. Some oil suppliers will state this on their product data sheet whilst others will keep it a closely guarded secret (at least until the lab does a test on it). New oil values for car motor oils tend to be between 7 and 12 and you would often say a drop of 50% from the new oil is the time to change.

Oxidation, nitration and sulphation – these tend to be an indicator of the oil degrading as oxygen, nitrogen and sulphur react with it during the combustion process. It helps to have a new oil and typically 20 above this would be an alarm level, but some new oils can have starting points of 50+ depending on the additives. So use trending and the TBN data in combination with new oil data for comparison.


This section is basically a list of the common things that can go wrong with your engine and is why we look for them.

Fuel – all engines will introduce fuel. There is no hard and fast limit with a car as it really depends on driving style and if it’s petrol or diesel. Typically less than 2% is normal though in diesel and a bit higher for petrol engines. Major fuel leaks you may see values in excess of 15%.

Coolant – people often think of a coolant leak as “my head gasket has gone” suggesting it’s some switch from perfectly fine to major failure. In fact coolant leaks are actually quite a slow process and can develop over years with very tiny leaks first. The failures you know of are only after a long period of failure occurring and detectable by oil analysis throughout this period. You will often see sodium and/or potassium in the oil rather than glycol as it boils off quickly or is broken down to various glycolates in the engine. This means all you see are the major additives left behind after the coolant boils away, namely sodium and/or potassium which are corrosion inhibitors designed to stop your coolant rusting your engine.

Grease – this is rare but if you see lithium this means you have grease getting into your oil. Potentially from a pre-greased part.

Dirt – this can be silicon as in sand or aluminium silicates as in dirt. Silicon and aluminium in an approx 2:1 or 3:1 ratio are a classic sign of dirt ingress. There are two potential sources of dirty oil or dirt through the air intake. You can identify the source often by looking what’s wearing. So if the upper cylinder is wearing then it’s likely air intake and lower cylinder it is likely the oil was dirty or a dirty top-up can was used.

Soot – if you have a Diesel engine your oil will be sooty, and much less so from petrol as it’s a cleaner burn because it’s spark rather than compression ignited. Soot is produced when the fuel doesn’t burn fully leaving behind tiny carbon particles we call soot. So this usually is only a Diesel engine parameter to worry about. Soot around 1 to 2% is normal for an end of oil life. 4 or 5% might suggest a blow by issue or you are not changing your oil often enough. As you go beyond 5 or 6% you risk entering a gelling scenario. Gelling is when the oil has so much soot that when heated it turns solid and can soon lead to a failure of an engine.

Water – water can be from coolant or simply condensation from not driving enough to boil it off. We measure both free (as in not dissolved and separates easily) and dissolved water. Dissolved water may sound strange as oil and water don’t mix, but the additives in the oil basically contain the oil equivalent of your average washing powder to get everything nice and mixed up. Typically values less than 0.2% or 2000 mg/kg are normal.


First and foremost the biggest question you will have is what’s normal for wear as everything wears and it needs to take into consideration a few factors.

Alarm limits traditional approach

I have put a link to some generic road vehicle limits lower down in this paragraph. These are typically for trucks but you can really factor to any engine by just adjusting for the volume of the engine. If you have a small engine with not a lot of oil you will have much more concentrated wear than a large marine engine with a very high oil to component in contact with the oil ratio. Hence the smaller the engine the higher the point you start alarming. So if you have a truck with say 25L sump which we can assume the alarm limits link below is based on and your car only has a 5L size then potentially you could have 5 times these limits before a problem occurs. It’s not as straight forward as this though as the the style of driving changes too. So it may only be double. In fact most labs need to develop what is called a trend to compare your wear rates over time to answer this question. This doesn’t help you if you have only ever had one sample so far. That is why we invented a test called LubeWear to make it easier.

Alarm limits easier way

In the wear section you will see two of every wear metal e.g. two iron, two aluminium etc. One is the ASTM method which is what every other lab uses and the other is LubeWear. The ASTM method only detects tiny normally sized wear particles, but doesn’t detect the big large wear particles. You can read about this here if you are interested in why this is. LubeWear detects the small normal and the large abnormal wear. This means you can much simply diagnose using a comparison. If LubeWear is about the same as the ASTM method then this means the wear is normally sized and is likely just down to time in use. However, as the LubeWear value increases above the ASTM method you are looking at abnormal wear particles. So in simple terms if the values are the same that’s good, if the values are different that is bad and the bigger the difference the worse it is. So what do the wear metals mean?

Pistons – these are Aluminium

Rings – usually Chromium or Molybdenum

Liner – often seen as iron

Bearings – copper, lead and tin. You may also in rare instances see aluminium or silver bearings too. Lead on its own can also be from the fuel used if leaded in classic cars.


Oils wouldn’t be oils without additives to remove unwanted properties and add desirable ones to the oil. Typically to have an effect most additives will be added in high 10s, 100s or even 1000s of ppm.

Detergents and dispersants – your engine produces a lot of nasties and the air pollution laws mean it can’t go out the exhaust, so this means it has to go into the oil instead. Hence your oil has oil equivalents of washing up powder to keep everything clean and nice. The typical additives are calcium and magnesium and can be anywhere between 1000 and 5000ppm depending on the oil in use.

Anti wear – your oil has special additives called anti-wear additives that react with surface components to form hardened layers of metal. The most common one is ZDDP containing zinc and phosphorus and importantly sulphur which is the active part of the molecule. This can be anywhere from near nothing to a couple thousand ppm. It really depends on the brand.

Friction modifiers – some oils will add molybdenum disulphide that acts like graphite to cause a solid lubricant film on components. It is a popular additive used anywhere between 40 and 200ppm. The problem is it’s use has rendered detecting molybdenum ring wear near impossible using traditional means from a lab perspective and it’s only with LubeWear comparison we can now differentiate the two.

Antifoam – with an oil full of a washing-powder- like-substance it’s not surprising it could foam. However the oil companies have a solution for that and they use silicone oil at anywhere between 10 and 40ppm to reduce the effect. The problem is then determining if the oil is dirty or anti-foam with silicon present. The answer comes from looking for wear. If there is none it is likely to be additive rather than contamination.


This article will not teach you everything you will ever need to know about oil analysis. You can read the rest of our training portal to start that journey, but this will give you a great start.

I have diagnosed over my career close to 1 million samples and I’m still learning new things. So don’t expect to be able to do it all on day one. You can certainly get a good feel for reading reports very quickly and if you are stuck that is why we put comments on the top of every page. Equally, you can always email or call to ask questions.

If you would like to reach out and ask a question about your report please use the contact us button below to do so.

Equally, Peter if you are reading this and think we have done enough to earn a 5 star review upgrade from 4 star now then please feel free to upgrade the rating 😉



Shortly following this article, I had an email from Peter.

Morning Adam, you’ve certainly gone above and beyond in your assistance. I’ve bumped up my review as it’s well deserved. Good to see a business so focused on customer service and being so proactive. 

Looks like another happy customer. If you have any questions following your report, don’t forget to get in touch and maybe we can post an article for you too to answer your questions. Out of interest below is the updated review.