Why is it possible to extend the synthetic oil change interval?

It’s generally understood that the synthetic oil change interval can be extended compared to regular oils; but what should the correct synthetic oil change interval be?

We all understand that we need to change oil: Whether it’s in the chip fryer or in our car engines! But What is the correct synthetic oil change interval? And why does it need changing in our cars?

Why is the synthetic oil change interval important?

Strangely, the reasons for the need of a regular synthetic oil change interval are common to all oils; including the home chip fryer:

• Oxidation - This changes the composition of the oils, and when combined with other elements, often forms harmful acidic compounds. 
• Contamination - This is usually from external sources (Silica), but a as the mechanical components wear, the wear particles also contaminate the oils. 

During the km’s between the synthetic oil change interval, the lube in your cars engine gradually becomes contaminated with silica, metal particles, gasoline and other impurities. As mileage accumulates, so do the contaminants. Eventually, enough impurities accumulate that the filter cannot remove them and the oil cannot keep them in suspension.

It’s obvious that these conditions can reduce the performance and life of an engine, and the way to control this is to change the oil when it no longer performs as it was designed to. The question is when does it reach the point at which it should be replaced?

Car manufacturers specify the synthetic oil change interval; usually between 10 and 15,000 km’s (Or 7 to 12 months) for normal operating conditions. Once again this is very vague. Surely if I use my car as a run about, driving very lightly to the shops and back once a day, the lube life should be extended? Unfortunately not!

Here’s a list of driving conditions that are deemed to effect oil changes:

• Repeated, short distance driving. 
• Extended periods of idling or low speed operation.
• More than 50 percent driving in heavy city traffic during high temperatures over 28 degrees c. 
• Driving for a prolonged period of time in cold temperatures and/or extremely humid climates. 
• Driving in dusty conditions or sandy areas. 
• Driving in areas where salt or other corrosive materials are being used. 

 With all these variables it’s obvious that determining the best synthetic oil change interval will not be easy. In actual fact as the engine ages so the ideal change intervals reduce, making the exact determination virtually impossible.

How to determine the best synthetic oil change interval.

One sure way to determine the best synthetic oil change interval is to have a sample of the oil analysed at pre-determined mileages, so as to predict the remaining life of the lubricant, additive package as well as determining contamination and wear rates. This is often used by heavy vehicle fleet operators that have to control running and maintenance costs; however it’s not that practical for the average car user.


Another way of trying to predict the synthetic oil change interval is by using the Total Base Number (measurement of the capacity of engine oil for neutralising strong acids. Note that this has nothing to do with the viscosity) to calculate the theoretical lifespan. There are a number of formulae, such as this one by Ted Kublin:

This formula tries to rationalise the effect of performance and driving conditions on oil life.The TBN used, is more of a constant ranging from 8 to 12 for synthetic oils, and around 4 for mineral based oils.

Surprisingly tests carried out using a popular synthetic motor oil, confirm the synthetic oil change interval when compared to the oil sample being analysed. Both results put the oil life of a typical 6 cylinder petrol engine, with a mix of driving conditions, at over 30,000km’s. So this is two to three times the recommended lube intervals that the OEM’s recommend for the engine!

Although synthetic oils are still more expensive than mineral oils (The price difference is reducing), their performance is undoubtedly superior. The synthetic oil change interval is extended because synthetic lubricants are engineered to:

• Reduce friction (Thereby reducing temperature and wear rates while increasing power output) 
• Their superior additive packages mean that oxidation and sludge build up is reduced. 

The only contaminant that cannot be controlled using synthetics is the silica ingress. If it weren’t for this the synthetic oil change interval could effectively be 40 - 50,000km.

So to come back to the original question: What should the synthetic oil change interval be?

It might be safe, under normal on-road conditions, to extend a synthetic oil change interval up to 20 to 30,000 km’s. However the OEM service policy must at all times guide the decision on when it should be changed; This has legal implications that could inadvertently cost you a lot of money.

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