In the previous topics that tackle oil consumption, it’s been stated many times that there is no such thing as a complete absence of oil consumption. Our expert consultant, Stevan Dimitrijevic explores this topic in depth, explaining why this is just a myth and why the dipstick might show the same oil level throughout the whole oil change interval.
Every internal combustion engine consumes oil to a certain extent, depending on its construction. Some engines, like the rotary (Wankel) type, are pretty heavy oil consumers, but these are quite rare on the market.
From a historical perspective, oil consumption dwindled over time, so that topping up oil between two oil changes became unnecessary at some point. This especially applies to the period of the eighties when average oil consumption was below 1 gram per kWh, and in many cases even below 0.5 g/kWh. With such oil consumption, cars that were driven mostly in cities and on regional roads often consumed around 50g (30-80g) per 1,000 km. Consequently, with the oil change intervals being 5,000 to 10,000 km (typically 7,500 km), many vehicles didn’t require topping up, and if they did, it was a very small amount. If we add to that the fact that oil level measurement methods were not so precise back in those days and that there were many factors that could mask the actual oil consumption, one could easily conclude that the engine did not consume oil at all. Such misconceptions were further deepened by the generally accepted opinion that oil should be changed in spring, which was only partly true for that period. But for modern cars, especially those that go many kilometers in one year, this is completely erroneous.
Even today, many drivers would swear their engines do not consume even the smallest drop of oil, which is a typical misconception.
Factors leading to such erroneous conclusions were most often: unburned fuel mixing with the engine oil (up to a few percent) and a certain amount of (water) condensate. These compensated for the few dl of oil consumed within a short change interval, creating the impression that the oil level has not “moved a bit”.
Just as any misconception, this one too was only seemingly grounded, and was even more widespread in rural areas and developing countries (with not so many highways) where driving conditions were not severe (short trips at around 100 km/h are not the same thing as driving on a highway for several hours at the speed over 120/130 km/h).
Since the eighties, engines have consumed less and less oil, but at much smaller progression. Nowadays, 0.1-0.2 g/kWh is considered a usual consumption, even though there are exceptions that go far beyond these figures. However, the need to top up oil between two oil changes became bigger because of the (multifold) increase of the oil change intervals. Nowadays, 15,000 km is considered to be a minimum oil change interval (in normal driving conditions), whereas 20-30,000 km is the most frequent interval, and there is also that of 35-50.000 km (mainly flexible).
At “shorter” intervals (typically 15,000 km) adding 1 liter of oil as well as the absence of the need to add oil is usually considered normal. For longer intervals (20,000 km and more; 350 working hours and more) normal means adding 2 or even 3 liters (in any case half of the oil tank capacity).
A modern edition of the “zero oil consumption” phenomenon is typically this: the oil change interval is recommended at 20 or 30 thousand kilometers, but you change it at 10 or 15 thousand. You drive easily and fuel-efficiently in light city traffic or suburban roads. Most often, the engine does not get to reach the working temperature so there are a lot of condensates, and the oil evaporation is minimal. The overall consumption is 0.5 to 1 liter, but because of the aforementioned factors, you usually don’t even notice it.
It’s interesting to mention two typical cases when an increase in the engine oil consumption can be expected, both being only temporary, and the causes being similar to those that lead to “zero oil consumption”.
1) Switching from lower quality to higher quality oil, typically from synthetic-based to synthetic. Viscosity grade is consequently lower, at least judging by the W mark. Over time, certain deposits begin to accumulate, especially when mineral oils are being used and/or in case of an irregular oil change, creating some kind of sealing because the deposits in the combustion chamber are on the pistons and piston rings. A higher quality (synthetic) oil that is rich in detergents will gradually dissolve and disperse the deposits which will enable better (normal) oil flow, and sometimes even higher consumption (more oil ends up in the cylinder). In such cases, oil consumption usually reverts to normal and stabilizes at the second or at most third oil change. When it comes to mineral oils, somewhat poor oil flow and deposits can lead to oil consumption that is less than what is considered normal, which some drivers even consider as an advantage - the truth being quite the opposite.
This is a case when we must say - it’s better for the engine to consume small amounts of oil than NOT to consume at all (illusory “zero consumption”) because poor lubrication can harm the engine.
It is an interesting fact that many car owners refuse to switch to higher quality synthetic oil and are ready to “bet their lives” that “this synthetic stuff is bad for older engines”. Synthetic oils as such have no effect on oil consumption and the sealings (the opposite being another misconception) and can be freely used in older engines. Of course, if an engine has reached its peak-life phase, there is not much economic justification to use synthetic oils, but that doesn’t make them bad for the engine.
2) After a long period spent in city traffic, especially during winter, unburned fuel and water condensate set inside the oil tank. If after such a period one goes for a few-hour drive outside the city, especially on highways at the maximum speed allowed (130 km/h), oil consumption can be enormous! However, that’s not just oil - it is also fuel and water that evaporate. Nevertheless, a great amount of oil does get consumed, in many cases more than what is considered usual. Upon finishing the drive, it’s a good idea to check the oil level. If it’s minimum or below the minimum, a certain amount should be added. After some time spent driving again, things should get back into normal. If they don’t, it could be that the winter conditions in urban zones have degraded the oil quality, so you will need to change the oil completely. Replacing oil and oil filters will then make consumption normal again.
In the end, there is one more thing to highlight: new cars (after the car’s engine is fully developed) consume less oil (approximately, in the first 100,000 km), and after that, they begin to consume more oil. The engine parts simply wear and seals become loose over time (at first these are only tiny cracks where you don’t see a greasy stain under an engine). If your engine consumes a little oil - that is o.k. but don’t expect it not to consume at all because that is simply impossible. In summer, consumption is somewhat bigger due to oil evaporation. If you drive on highways and mountain roads where the engine is under bigger pressure, it’s expected that the engine should consume more oil. If the oil consumption is stable and moderate, it means that the engine is in good shape. Any anomaly (higher or sudden “zero” consumption) is a reason for you to visit an authorized car service to establish the reason.