Synthetic engine oil can provide excellent resistance to heat, oxidation, deposits and viscosity breakdown, but that does not mean every synthetic oil should be changed at the same interval. The correct oil-change interval depends on the vehicle, the lubricant being used and the conditions in which the engine operates.
For most vehicles, the best starting point is the service interval specified by the vehicle manufacturer. However, some premium synthetic lubricants are specifically formulated and warranted for longer service intervals, while severe operating conditions can require oil to be changed sooner.
There is no universal kilometre figure for synthetic engine oil. Follow the vehicle manufacturer's requirements unless the lubricant manufacturer specifically permits a longer interval for your application.
Why There Is No Universal Synthetic Oil-Change Interval
The word synthetic describes how an oil is formulated, not how long it can automatically remain in service. Two oils of the same viscosity can use very different base oils, additive systems and performance specifications, giving them different levels of resistance to oxidation, deposits, viscosity change and contamination.
Engine design also plays an important role. Oil operating in a naturally aspirated engine with a large sump and mostly highway use may experience very different conditions from oil in a turbocharged direct-injection engine that spends most of its life making short trips.
This is why an oil-change interval should be based on the complete application rather than a simple rule such as "synthetic oil lasts 10,000 kilometres."
What Determines an Oil-Change Interval?
Engine oil gradually accumulates contaminants and its additive system is consumed while the oil is in service. The rate at which this occurs depends on several factors.
- Vehicle manufacturer requirements – The manufacturer specifies servicing intervals based on the engine design and intended operating conditions.
- Oil specification – API, ILSAC, ACEA and manufacturer specifications define important performance requirements for the lubricant.
- Oil formulation – Premium synthetic oils may provide greater resistance to oxidation, deposits and viscosity breakdown.
- Engine design – Turbochargers, direct injection, emissions systems and sump capacity can all influence oil stress.
- Driving conditions – Short trips, towing, dust, prolonged idling and high loads can accelerate oil degradation.
- Contamination – Fuel dilution, soot, moisture and combustion by-products can reduce the useful life of the lubricant.
Normal Service vs Severe Service
Many vehicle manufacturers publish separate servicing requirements for normal and severe operating conditions. Severe service does not necessarily mean racing or extreme off-road use. Some very ordinary driving patterns can place additional stress on the oil.
Examples can include frequent short trips, extended idling, repeated towing or heavy loads, operation in dusty environments and sustained high-temperature use. Vehicles used commercially may also accumulate engine hours much faster than their odometer suggests.
Always check the service schedule for your specific vehicle. A manufacturer may classify operating conditions as severe even when the vehicle is being used in what appears to be normal everyday driving.
What About Oil-Life Monitoring Systems?
Many modern vehicles use an electronic oil-life monitoring system to help determine when an oil change is due. These systems generally calculate remaining oil life from operating data such as engine temperature, load, running time, starts and driving conditions.
An oil-life monitor should not be confused with a laboratory oil analysis. In most vehicles it does not directly measure the chemical condition of the lubricant. Instead, it uses an algorithm developed for the vehicle to estimate when servicing is required.
When the vehicle manufacturer specifies that the oil-life monitor should be followed, it forms part of the manufacturer's servicing strategy.
Can Synthetic Oil Last Longer Than Conventional Oil?
A high-quality synthetic oil can generally resist oxidation, high temperatures and viscosity breakdown better than a conventional mineral oil. This gives lubricant manufacturers greater scope to formulate oils capable of longer service intervals.
However, the ability of an oil to remain stable for longer does not automatically override the vehicle manufacturer's servicing requirements. The lubricant must also remain suitable for the engine, operating conditions and applicable warranty requirements.
Where extended drain intervals are desired, the oil should be specifically designed and recommended for extended service rather than assuming that any product labelled synthetic can simply be left in the engine longer.
AMSOIL Synthetic Oil-Change Intervals
AMSOIL offers several passenger-car engine-oil ranges designed for different service requirements. This allows motorists to choose between conventional manufacturer-recommended servicing and products specifically engineered for extended drain intervals.
AMSOIL OE
AMSOIL OE 100% Synthetic Motor Oil is designed for motorists who intend to follow the oil-change interval recommended by their vehicle manufacturer or oil-life monitoring system.
AMSOIL Extended-Life
AMSOIL Extended-Life 100% Synthetic Motor Oil is designed to provide additional service life and is suitable for extended drain intervals of up to 32,000 km or one year under qualifying normal-service conditions.
AMSOIL Signature Series
AMSOIL Signature Series 100% Synthetic Motor Oil is AMSOIL's premium passenger-car engine oil and is designed for extended drain intervals of up to 40,200 km or one year under qualifying normal-service conditions, or up to 24,140 km or one year under qualifying severe-service conditions.
Extended drain recommendations are subject to AMSOIL's current product instructions, application requirements and service limitations. Always check the current recommendation for the particular product and vehicle before extending an oil-change interval.
Should You Change Synthetic Oil Earlier Than Recommended?
Changing engine oil earlier than necessary is unlikely to harm an engine, but it can increase maintenance costs and generate unnecessary waste oil. A good synthetic lubricant does not suddenly become ineffective simply because it has reached an arbitrary round kilometre figure.
There are situations, however, where an earlier oil change may be sensible. Fuel dilution, coolant contamination, excessive soot, mechanical problems or unusually severe operation can shorten lubricant life considerably.
For heavily modified, competition or specialised engines, used-oil analysis can provide much more useful information than relying on a generic kilometre interval. Laboratory analysis can help determine viscosity, contamination levels, wear metals and the remaining condition of the lubricant.
Does Synthetic Oil Still Need to Be Changed Once a Year?
Low annual kilometres do not necessarily mean engine oil can remain in service indefinitely. Moisture, fuel contamination and combustion by-products can accumulate even when a vehicle travels relatively little distance.
For vehicles driven infrequently, the manufacturer's time-based servicing requirement should still be considered. Many lubricant and vehicle manufacturers specify an interval based on both distance and time, with whichever limit occurs first determining when the oil should be changed.
What If My Vehicle Is Under Warranty?
When a vehicle is within its manufacturer warranty period, the safest approach is to use an engine oil that meets the required viscosity and lubricant specification and to follow the servicing requirements published by the vehicle manufacturer.
Using a premium synthetic lubricant does not remove the need to maintain appropriate servicing records or comply with applicable warranty conditions. Extended oil-change intervals should only be used where they are appropriate for the vehicle and supported by the lubricant manufacturer's current recommendations.
Choosing the Correct Engine Oil
Oil-change interval is only one part of selecting the correct lubricant. The oil must also have the appropriate viscosity and meet the specifications required by the vehicle manufacturer.
If you are unsure what specifications such as API, ILSAC or ACEA mean, visit our Engine Oil Standards Guide. You can also use the AMSOIL Vehicle Finder to identify suitable AMSOIL lubricants for your vehicle.
Do not choose an oil-change interval simply because the oil is synthetic. Start with the vehicle manufacturer's requirements, consider how the vehicle is actually used and follow the service recommendation of the lubricant you have selected.
Oil Information & Technical Guides
Explore our oil knowledge base with practical guides covering oil standards, viscosity, base-oil technology and application-specific specifications.
