How many kilometres should we drive between oil changes? 3000? 5000? 10,000 kilometres? They may all be correct. Short trips, cold weather or heavy loads shorten oil life. Extended highway driving doesn't break engine oil down as fast, so it really depends on how the vehicle is driven.
Some drivers still believe in oil changes every 3000 kilometres. Owner's manual may list oil changes every 10,000 kilometres or even more! In recent years, manufacturers are incorporating programs on vehicles that will monitor oil life for owners. While many new vehicles monitor oil life, General Motors has been a leader in this area with oil life monitoring incorporated even on some 1996 vehicles. For 2003, almost all GM vehicles now have oil life monitoring for both engine and transmission oil. In the near future, these oil life monitoring systems will also be linked to vehicle maintenance schedule intervals. Oil life monitoring is a programmed calculation in the powertrain control module based on engine revolutions. As the engine runs, the calculated number decreases until it is zero. Then the computer turns on the "change oil" light or message. Other computer inputs will decrease the oil life number to zero faster than normal. Cold engine temperatures allow moisture, acids and unburned fuel to contaminate the engine oil faster, so engine temperature is used as an input. Engine load is calculated based on intake airflow, throttle position and engine vacuum. Light engine loads cause the oil life monitor to decrease at normal rates, while increased loads cause a faster decrease in calculated life. So what makes oil wear out? Oil is a complex combination of hydrocarbons and additives. The oil itself is made up of long chains of molecules that can be "sheared" by mechanical motion inside the engine. The oil doesn't really wear out, but it's ability to withstand loads and separate moving components without allowing metal to metal contact is decreased as the molecule chains become shorter. Additives, used in the oil to increase viscosity, prevent corrosion, suspend particles and prevent foaming, do wear out. Oil is exposed to high temperatures at the top piston rings causing additives to break down. Acids formed by water condensation in the crankcase and fuel dilution during cold starts also break down the additives. Dark or thick engine oil are signs of engine oil additive breakdown. It's time to change oil and filter. Synthetic oil poses another problem. Because synthetic oil doesn't break down the same as regular oils, it can be used in the engine longer. However, contaminants still form in synthetic oil so it needs changing on a regular basis too. Oil life monitoring system programs are designed to calculate oil life based on the oil originally specified in the owner's manual. If the engine is filled with synthetic oil instead of regular oil, the oil life monitors have no way of knowing this and it isn't possible to change the programming for the calculations. The "change oil" indicator comes on before it is necessary to change the synthetic oil. Of course, installing regular oil in vehicles originally equipped with synthetic oil is not recommended but if it did happen, the opposite would happen: the "change oil" indicator would not come on until long after the oil really needed changing. A vehicle with synthetic oil used in warm weather, light load highway driving could possibly be driven 16,000 kilometres before an oil change is needed, but regular oil used under the same conditions may need changing at 10,000 kilometres. Dust is another variable to consider when determining oil change intervals. There are no sensors measuring dust entering an engine. Today's air cleaners and duct work seal well but vehicles driven in very dusty conditions can still have a little dust leak in. It only takes about 28 grams (one ounce) of dust at the piston rings to wear them out so it is fortunate that most dust entering the engine goes right back out the exhaust or is trapped in the engine oil. Vehicles with intake air dust leaks or those used in dusty conditions should have oil changed more often than oil life monitoring systems would tell us. How many kilometres should you drive between oil changes? It all depends on driving conditions. One thing not to forget however, it to check oil levels often, regardless of oil change intervals.
