For Diesel Users

Since its launch OIL SPY has been developed for a number of industries, not just for cars but aircraft and of course plant with diesel engines in which it works admirably. Naturally there are many diesel cars, trucks and 4WD vehicles as well, and we are including the above chart as an example of how the oil test will look as it is a bit different to the petrol result. OIL SPY has a number of tasks to perform.

1) To identify contaminants such as fuel dilution, which is the same result as the petrol version with a yellow corona around the outside of the stain.

2) Coolant ingress into oil which is again the same result as the petrol version, showing jagged edges around the stain (some say it looks like the rings in a cut down tree), that is water in the oil.

3) Soot… This is really the vital one and the most interesting for engines today. Apart from cooling and lubricating moving parts in the engine, the oil has to act as a carrier for the inevitable and essential by-products of combustion. We’ll call it soot. This has to be carried uniformly in the oil and the detergents in the oil ensure that it is carried in this way. The oil is a reservoir for this dirt and the more that emissions are reduced the greater the amount of soot/debris that has to be contained in the oil. As always there is a ‘but’, in this case the ‘but’ is that everything has its limits, and it is not related to total numbers of hours but how the engine is used. Low power, idling high power, low load, high load, hot or cold ambient temperature, all these affect what is called the oil’s dispersancy, its ability to carry the soot around. Once it is overloaded, it’s time to change the oil before sludge develops, (ie straight away). The diagram above shows typical examples (note how the samples are much blacker for diesel than for petrol engines compared to the pictures on our instruction sheet).

The “green” examples show good dispersancy with all the soot wicked out to the edges of the stain.

The bottom left (yellow) square shows the oil has uniformly spread out to the edge of the stain (blot). There is no evidence of a lighter colour or rings at the edges, this shows that the oil is dispersing properly and that all the foreign bodies are held in suspension. It is in the marginal amber box as it is very, very black and dense and so is nearing overload (but still working properly) and should be monitored regularly. Check for a sticky centre which is further evidence of oil break down. Top right corner (yellow) shows a central spot developing, showing that the dispersancy of the oil is just starting to fail.

All the red squares are obvious showing that the oil can no longer carry all the soot and is depositing it in a sticky spot in the middle. Its ability to disperse the soot has reached saturation.

So What? Don’t all diesel engines have dirty black oil? Yes, the oil gets dark quickly but no, when it gets to the red square stage: the oil change is (past) overdue. At this stage the sludging can block the flow of oil around the engine (with obvious results, especially when it’s cold) and can adversely affect the other additives in the oil.

We should also mention Exhaust Gas Recirculation (EGR). Many engines are fitted with this system (often troublesome on certain marques). They effectively send the emissions back to the combustion chamber creating a multipass opportunity for soot to ingress into the lubricant. An EGR valve regulates how much exhaust gas is re-circulated. For example at idle 70% of the gas is re-circulated whilst this is only 10-20% at full load. Whilst EGR does reduce NOx levels to the atmosphere, soot loads in the lubricant can be expected to increase dramatically, causing increased temperature and viscosity, dispersancy failure, fouling, deposits and wear. The internet is full of cases where engine failure has occurred through sludging (dispersancy failure) inside vehicle warranty and inside oil change intervals. Some manufacturers have claimed that not changing the oil in time is outside of the warranty as it is the owner’s responsibility to carry out this essential maintenance. Be warned… this is a hot topic at the moment that will only increase in temperature and complexity as emission tolerances are reduced further.

2 comments about this article.

  1. Steve says:

    On you say “More soon…”. That was almost a year ago. Do you plan to release a diesel specific Oil Spy kit? I have a Land Rover Defender 200TDI and a Skoda Fabia TDI (PD engine).

  2. Hello Steve,

    Please email your question to and our technical team will get back to you.

    Best wishes,

    The Oil Spy Team

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