Oil Aging Explained

In this article...

  • Oil Aging and Degradation: Why do I need to change the oil?

  • If I drain old oil from my engine, it seems thick and black, why?


Oil Aging and Degradation: Why do I need to change the oil?

The reason is two-fold:

To drain out oil that is no longer useable.
To drain out any build-up of contamination.

The actual Oil Change Interval (OCI) varies, but a lot depends on the use of the machine or in the case of vehicles, the driving style as to exactly when it needs changing.  For simplicity, most manufacturers now suggest 12k miles or once a year as opposed to every 6k or even 3k miles that used to be the norm as recently the mid 1990s in the UK for passenger car engine oils.  Oil will age irrespective of whether it is used or not.  In fact, short trips or short run periods in cold weather will harm the oil more than extended run-time or trips will.  Extended runtimes allow much of the harmful build-up of moisture, unburnt fuels and acids to evaporate from the oil. 

Many modern cars are now able to utilise an algorithm in the ECU to determine the required oil change interval based on a number of factors such as duration of use, ambient temperature, speed and acceleration.  Owners that drive slower and/or for longer will have longer OCI’s than owners who drive faster and/or on shorter runs without full warm-up.  Some executive cars are fitted with oil condition sensors that measure the true state of the oil rather than hazarding a guess.

However, with older classics, the simple "Oil Spy" type test as used by mechanics over the years will identify when the oil is due for a change.  But, your classic car’s handbook states every 3k miles or possibly every 6k miles.   Which is correct for that era when the oils available weren’t to the same quality as those available today.  As late as 1980, the MGB handbook for the B Series engine recommends an OCI of 6k miles and six months whichever came first whereas later in the 1990s the A Series was 6k miles or 12months, whichever came first. 

So can this OCI on my classic engine be extended? 

In theory, yes; 

  • If an engine is running on synthetic oil then less wear is taking place so then the build up of contamination is lessened. 

  • The base oil will be longer lasting and remain in specification for longer despite the temperature stress.

  • There is less corrosion risk from the lack of Sulphur impurities in the synthetic base oil.

  • If you are using an updated fuel injection system and electronic engine management ignition system there will be less build up of fuel dilution and harmful combustion by-products as a result of a poor state of tune.

What must be remembered with US engines is that the cost of oil in the US market was insignificant and so even as recently as 2004 with the MG ZT 260, we saw OCIs of 5k miles on the Ford V8 used in the ZT260.  So whilst European and Japanese engines have much longer OCIs, the oil specified is of a much higher grade at a higher cost.  But it is swings and roundabouts, you pay more but less frequently.

If I drain old oil from my engine, it seems thick and black, why?

Oil, like fine red wine, ages and oxidises.  The process of oxidation is naturally occurring in the oil, but the rate at which this happens depends on the following factors:

Temperature – the higher the temperature, the shorter the oil life.  Based on the Arhenius Rule, for every 10°C increase in temperature above 40°C, the oil life is halved.

Moisture – the more water present the faster the oxidation rate

Oxygen – the more oxygen from splashing induced by low oil levels the faster the oxidation rate

Catalytic reactions – most typically from copper and other wear debris particles

The condition of the engine – ring wear, fuelling etc

The quality of the base stock oil in terms of the levels of impurities

The quality of the additive pack in terms of the anti-oxidants available to slow down the oxidation rate of the oil.

As the oil oxidises, there are two parallel problems arising. 

  • Firstly, as a result of the oil molecules reacting and joining together, the oil becomes thicker and darker.  The heavier the molecules become, the heavier, or thicker, the oil gets to be.

  • Secondly, a by-product of the oxidation is the formation of acids, which increases the risk of corrosion of component surfaces.  Examine an old engine and look at the chemical damage that is visible on the surface of the component.  It is always advisable to change the oil prior to storing a classic for Winter rather than at the end of storage.


Figure 2 - The Life Cycle of Oil - Ideally oil will be changed when the maximum useful life has been attained (condition-based) rather than by a schedule where the oil may be still fit for purpose or even beyond its use-by-date.

In order to counter acid build-up from combustion by-products and oxidation, some of the additive package is over-based in order to neutralise these harmful acids occurring in the oil.  One of the primary sources of acid in engine oil is usually the result of fuel dilution.  Fuel also contains Sulphur impurities and this can react with water from combustion and condensation to form Sulphuric Acid.  However, the level of Base Reserve in the oil depletes with time as it neutralises harmful acids and hence this is another reason to change the oil to ensure there is always sufficient Base Reserve to counter acid contamination.

Remember that when you drain an engine oil, there is as much as 20% oil remaining residual in the block and cooler, and just 10% residual of severely degraded old oil will damage the new oil’s additive package within hours.  So if you have missed a service, have the oil changed, then run the engine for some time (50 miles or so) and then change the oil again.

Older engines running on carburettors and points-based distributors or injector pumps generally have less efficient combustion than their modern counterparts.  The engine goes off tune after a period of running until its next service with new points and plugs.  Therefore the contamination from combustion on the oil is proportionately higher than on modern engine oils.  Allied to better quality, cleaner fuels and base stocks and adpacs, the OCIs have increased significantly to as much as 30k miles and 24mths on some cars, albeit with a need for very expensive synthetic oil.