Engine Oil Samples For Pre-Purchase – Are They Really Useful?

After years of taking oil samples for pre-purchase inspections, I’ve noticed there is a lot of confusion in the industry on what can be expected from doing a pre-purchase oil sample. The common misconception is that an oil sample is going to tell you exactly how worn the engine is, and absolutely everything wrong with it internally. I had a look at our records, and over the years we’ve done over 600 oil samples for our clients. After this volume of results, you start to see common results and trends.

What is an oil sample?

An oil sample is when a small portion of oil is taken from an engine, gearbox or piece of machinery, and sent to a lab for analysis. An oil sample is most commonly taken with a clean hose through a dipstick, although there are a number of different ways to take a sample. It is recommend oil samples are taken when the machine is hot and has been running previously. The easiest comparison is that it’s like a blood test for an engine. A sample oil analysis report can be found here.

How do oil samples help?

The truth is, an incorrectly interpreted oil sample can be completely useless. Further to this, a single oil sample can be taken out of context and there are best practices for interpreting the results. On the other side of the fence, an oil sample can be a life saver if it shows up with other contaminants that should never be in a component, for example: coolant or salt-water. Typically a one-off pre-purchase oil sample is not necessarily as useful for wear unless other previous samples can be used to create a trend, or there are significant amounts of different metals in fresh oil.

To get the most accurate results, the more you know about the oil the better. Taking note of the viscosity of the oil, make of oil, and length of time the oil has been in the component will put you in a better position to analyse the results. If the oil has just been changed, you may be sampling fresh oil. Do you have a number of previous oil samples over the life of the component? If so, you’re onto a gold mine of information and will be able to come to a well informed diagnosis.

When are oil samples a life saviour?

Let’s use a few case studies of scenarios I’ve encountered over the years to paint the picture:

Case Study 1

We had a pair of Cummins 6BTA diesel engines in a boat that had little service history. The boat was over 10 years old, and it had the 330hp variants fitted.

It was unknown when the oil was changed, although the last invoice dated 18 months prior – so we were assuming no-one had changed it since then.

An oil sample was taken, and there was evidence of salt in the oil, indicating there was saltwater getting into the engine. It was further investigated that the after cooler had failed and was putting pressurised saltwater into the engine!

The oil sample was a god send, because it found an unknown issue and the buyer was able to fix the after cooler before major damage was done. With that being said, if the engine had been serviced in its recommended time frames (we’re particularly referring to the saltwater service) this would have been picked up too, although if the cooling system had been flushed with a chemical like barnacle buster it would not be found.

Case Study 2

We inspected a 10 year old boat, with a pair of pod-type gearboxes fitted. The vessel had little information about it’s last 5 years of use and maintenance, and had not been driven much, if at all, with a low amount of hours showing on the clock. The oil on the gearbox looked clean, although very hard to tell from a small sample. Oil samples were taken for testing, and sure enough sodium was found (indicating saltwater ingress). This resulted in a major strip down and repair of the seals in the underwater components of the gearbox, including updating to newer versions of seals and seal rings.

In this particular case the gearbox was not showing any signs of issues yet, and it was purely the oil sample that picked up the issue. This is a rather typical outcome for boats that sit for years with little use. When they start being used, the exisiting problems come to light. Had this have been left, the owner would have done damage to the gearboxes which would have resulted in more expensive repairs, although there was already a reasonably hefty bill to repair the pods.

Case Study 3

A 5 year old vessel with Volvo D11 engines fitted and 600 hours on each. The oil had recently been changed, and upon receiving the results of the oil samples they were both (port and starboard engines) labelled “STOP” due to elevated level of molybdenum. In some cases, this can mean excessive ring wear, although in this case it was unknown which oil brand was in the engine and was most likely due to an additive used by the oil manufacturer. Since both engines came back with elevated molybdenum, the results are likely a result of the wrong brand of oil used, but the engines are still okay. The only recommendation would be to change the oil for the correct grade recommended by the manufacturer. This is an example of when an oil sample might create unnecessarily caution on an otherwise fine pair of engines.

Case Study 4

We had an 11 year old pair of Volvo D6 diesel engines fitted to a popular sports cruiser. It was known the vessel had been sitting for a period, before Volvo dealership had completed a saltwater cooling system service to both engines. Upon receipt of the engine oil samples, it was apparent that there was 60ppm of alloy on one side, and 36ppm on the other. The recommendation from the oil lab was to change the oil, and resample within 10-20 running hours to ensure the readings were heading in the right direction. After enquiry with a number of Volvo dealers, it was concluded that alloy readings are quite likely to rise after a cooling system service is completed, due to the casings of the components in the cooling system etc. In this case, the oil sample was most likely fine, but there was limited data to compare to which caused alarm for a pre-purchase inspection. In these circumstances I believe the readings need to be taken with a grain of salt, as there is nothing to compare to and it’s a known fact that these readings rise when certain work has been completed. This is a perfect example of why oil samples need to be evaluated in context, and the truth is sometimes all the relevant information isn’t available to make a completely accurate diagnosis.

Case Study 5

A pair of 15 year old Twin Disc gearboxes, fitted behind a pair of diesels in a 40ft boat. Samples were taken from both gearboxes, and returned with elevated readings of copper, iron and lead. After discussions with Twin Disc, it was noted that depending on the type of clutch plates fitted to the gearbox, they may contain traces of those metals as they wear. Further to this, the usage of the gearboxes can change the way they wear and metals found, as well as the length of time the oil was in the gearbox. In this particular case, both gearboxes came back with similar readings, and it was clear that the oil samples were less useful for indicating wear, unless a trend of previous oil samples over the course of a number of years and hundreds of hours was provided. The readings still alarmed the buyer, but this was an example of when oil samples can be less useful without correct experience.


Whilst oil samples can definitely be useful, they need to be taken into context. Much like receiving a bad blood test from the doctor, it doesn’t necessarily mean you’re going to drop dead on the day! All the factors of the sample need to be considered including how long the oil has been in the component, service history (recent work), if you have regular samples to compare, type of oil and external factors (for example: general external condition/corrosion of the engine). The more experience the person doing the oil sample has, the more likely you’ll get an accurate evaluation. Whilst some people expect an oil sample to tell them exactly how worn the engine is, how many hours it has left and exactly when it’s going to fail, unfortunately this isn’t the case, although they can be a life saver in some circumstances.

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