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I hope you don’t need to explain what engine oil is and what its role in the engine is. These are commonplace truths that everyone knows. But there are a lot of questions, disputes and gossip on the terms of oil replacement even among drivers with experience. There are several methods to calculate the ideal interval: mileage in miles, engine hours, change according to the calendar, the amount of used fuel. Let’s talk about these in more detail below. Read more: Best Synthetic Engine Oil.

Best Synthetic Oil

How to Calculate Your Ideal Oil Change Interval

There is not and cannot be a general formula for determining the optimal oil mileage in the engine, as this figure depends on many factors.

First, you should consider the recommendations of the engine manufacturer, as a rule, oil in powerful engines wears longer than in small cars. Each engine manufacturer specifies its optimal replacement interval, for example, for some BMW engines it is 20,000. Accordingly, the oils with approvals for these engines must retain their properties throughout this mileage.

Secondly, it is necessary to decide what to start from mileage, engine hours, or calendar period. Below we will discuss each of these calculation methods in detail – how to calculate and in which cases each of them is applicable.

Third, consider your driving style. When driving in the city and the constant long idle time in the scourge of modern engines traffic jams, the oil wears out much faster. As well as in extreme driving at high speeds, off-road, or when transporting heavy loads, when RPM is often overdrive.

Fourthly, we should take into account the quality of the oil itself – different types of lubricants demand different change intervals. If you get really confused, it is possible to change oil by a quantity of burned fuel. Every driver is guided in his own way, and every method is correct.

At the same time, don’t forget to assess the actual condition of the oil. Hopefully, you have a healthy habit of checking its amount before each trip. You should urgently change lubricant if it has thickened to the state of tar, or, on the contrary, has lost its thickness to an almost watery state.

What is the easiest way to orientate in terms of replacement time?

Manufacturers specify the recommended oil change interval for the engine in kilometers or calendar terms – six months, a year. Modern engines often have a recommendation for a longer interval of replacement, but in this case, you should also choose oils from the LongLife category. On average, it is better not to exceed 10,000 miles, but also this figure is very relative and depends on the factors listed above – conditions of use and time for which such mileage is accumulated.

Some drivers get 10,000 miles in a couple of months, while others do not even get 10,000 miles in a year. For understanding, we shall divide the modes of using a car into three big conditional groups:

  1. Long-distance use only – driving about 20,000 miles a year.
  2. Weekend use – hardly any use of the car on weekdays for commuting to work, out of town, and city trips on weekends, 10-20,000 miles per year.
  3. Car for short trips to work every day – about 10,000 miles per year.

When it is better to be guided by the calendar

For the first group one cannot be guided by the mileage for replacement, we look at the calendar – a year has passed, we change the oil, regardless of the current mileage, as it begins the process of natural aging, the properties are lost, in addition, with long downtime in the engine condensate is formed, which accelerates the oxidation and corrosion of parts. This also applies to those cars that have not been used at all for a year, even if the car has been parked in the garage, change the oil before use.

When it is better to be guided by mileage

In the second group of cars described there are almost ideal conditions. Average speed, no frequent idling in traffic – in this case you can look at the mileage and change oil in accordance with the manufacturer’s specified regulations – 10-15,000 miles. But I would advise not to drive more than 10 000 or just divide recommended by the engine manufacturer mileage by 1,5-2 – it is the safest for the engine.

If you look closely at the independent tests of the use of different oils, few manufacturers successfully cross the line of 10,000 miles, after that the processes of rapid deterioration of the composition begin, there are products of oxidation, depletion, acid increases, the alkali number drops, the kinematic viscosity increases. These indicators depend a lot on the oil composition, and we will touch upon this subject in detail further on.

When it is better to be guided by engine hours

It is most difficult to calculate the optimum replacement period for the third group, which operates mainly in the city and every day. Low speed, idling in traffic, stopping and starting at traffic lights. That is the actual kilometers may be short, but the engine is frequently idling, which is more ruinous for the oil, than driving on the highway at a steady speed. In such a case it is not reasonable to orient only on the mileage, as in such a mode the oil can withstand 5 or 7 thousand, depending on the quality of the lubricant itself, its capacity and the condition of the engine. In this mode, it is recommended to be guided by moto-hours. For this purpose, a meter of engine hours is installed, if the car is not already equipped with it. The device is connected to any circuit and is powered independently.

Simple arithmetic will help to calculate the optimal number of moto-hours. For example, if the recommended mileage from replacement to replacement for your engine is 15,000 miles, then at an average speed of 50 mph this distance will be covered in 300 hours. That is, and we take this figure as a basis. I think the calculation is clear, divide the mileage by the average speed and get the engine hours. Just substitute your figures in the formula and be guided by the result. On average, it is recommended to set a limit of 200-400 moto-hours, but this does not apply to driving with heavy loads – high speeds and revolutions. The number of engine hours also depends on the type of oil, we will touch on this subject later on.

Keep in mind that moto-hours is not an actual measure of how long the engine has been running. The higher the RPM, the faster the meter spins. That is, if you like to drive, you will run the hours much faster as if you were covering the same distance at a slower speed.

Changing oil according to the amount of fuel used

There is a widespread opinion that you should be guided by the amount of fuel combusted, this method has also a place. For example, you have consumed 1000 liters of fuel, so change the oil. But in this method, you need to be a big pedant, to write down the consumption or collect receipts to make an accurate calculation.

Another disadvantage – it is difficult to calculate the golden mean specifically for your car, as it is impossible to compare engines with different numbers of cylinders, displacement, and capacity, the costs will differ at times for the same mileage, while, as mentioned above, in more powerful and volumetric engines, and the aging of the oil is slower than in small cars, as high speed for the engine 2 liters and the engine 0.8 liters gives different loads, in the first case, the engine does not even notice it, in the second case, it can simply fail, though it eats less gasoline than a more powerful counterpart. That is, we start from the average consumption and calculate by it the optimum value. But it is also necessary to correct the driving style.

How often should you change oil with an aggressive driving style?
Aggressive driving style – high-speed driving, fast acceleration to the “red zone”, frequent transportation of heavy goods, and so on.

For understanding: in sports cars, oil is changed after every race.

If you are a fan of fast and sporty driving, you can rely on the above recommendations, but divide the above values by 1.5-2 and use only synthetic oils.

How often you need to change oil with different compositions

Among the requirements that the oils have API, there is also the number of moto-hours that the oil must withstand:

Oil type Moto hours
Mineral 150
Semi-synthetic 250
For synthetic oils
API SJ/SL 250
API SM/SN 300-350
PAO 350-400
Esters 400-450

The table clearly shows how much the oil change interval depends on its composition. Mineral oils are used very seldom lately, and even that mostly for rare cars, which are more idle in the garage as exhibits than used, so we will not focus our attention on them.

By the way, from this table it is well seen why it is better to rely on moto-hours when driving in the city. Let’s assume that you use SM/SN grade oil, its service life is 350 moto-hours. When driving in the city the average speed is about 30 km/h, that is the mileage in kilometers is a little more than 10,000. At the same time, the manufacturers of oils of this class recommend it for increased mileage – 15,000 and more. If the average speed is even lower, i.e. driving in the city is mainly in traffic jams, the mileage will be even less.

But let’s return to our compositions. Next on the list is a semi-synthetic. It is rather widely represented on the lubricant market and will hardly become as mammoth as the mineral, but it is gradually substituted by hydrocracking, which, though there are some disputes about it, is classified as synthetic.

Semi-synthetic formulations are more sensitive to mileage than synthetics and start losing their properties much faster. Mainly due to the mineral components in the composition. Depending on the producer, they may be from 20 to 70%. Semisynthetics may be supplemented with a quality package of detergent additives that won’t lose their properties even after 10 000, but the base itself cannot withstand the long load, which affects the kinematic viscosity – it begins to grow. With increasing viscosity the load on the parts increases, they overheat more, which in turn worsens the properties of the oil even more. Thus, I strongly advise against rolling semisynthetic oil over 10 000.

Things are much better with synthetics. As we see from the table, for long runs it is better to choose esters, or at least PAO. Each manufacturer indicates the definition LongLife in the oil description, what means the possibility of its application for long runs.

Consequences of overdue changes

If you delay one change, you may not even notice the consequences without disassembly of the engine, and the following time changes and usage of good oil will partly improve the situation inside the engine, but if you constantly conduct such experiment and re-rolling, you may wait for some consequences:

  • Contamination accumulation. Oil is gradually contaminated with combustion products, it is clearly visible on the dipstick, it becomes black over time, in addition, additives responsible for flushing the engine are produced. As I wrote above, by 10,000 mileage few oils retain an acceptable alkali number, and the acid increases. The additive package which washes the parts is exhausted and the amount of substances decreases. That is the oil loses its detergent and dispersive properties, does not dissolve impurities, and does not retain them in itself, and all these substances calmly move along the oil channels and with all their hearts harm all the details of an internal combustion engine, depositing on the surface.
  • The parts start to wear out. It happens for the same reason – the package of additives, responsible for the protection of parts against wear, is exhausted. If you analyze the rerolled exhaust, you will find a lot of exhaust products – silicon, metal, aluminum.
  • The viscosity increases. The reasons are the same – accumulation of contaminants, oxidation, disturbance of additives polymerization, responsible for maintaining stable viscosity. As a result, the oil circulates poorly through the system, and the engine occasionally runs dry. This can lead not only to the refusal to start in cold weather but also to the engine knocking if it already has a depletion and is not in the best condition. In addition, due to the increased viscosity, the oil film is thicker, it has already written above.
  • Rotation of crankpins. The thinner the oil, the higher the load on the crankpins. This causes them to overheat and rotate.
  • The turbocharger (if it has one) will wear out. Most of all its rotor suffers. The oil waste has a negative impact on the compressor shaft and bearing, scratches and damage appear on them. Oil with accumulated impurities clogs the ducts, which can cause the compressor to jam.

Oil change interval in a diesel engine

The same rules apply to diesel engines as to gasoline engines. The only difference is that the working fluid in them is exposed to greater external influences, for this reason, the oil change is carried out a little more often in a diesel engine. The frequency of changes is also affected by the quality of domestic fuel with high sulfur content, which also negatively affects the oil.

As with gasoline engines, take the engine manufacturer’s instructions and divide them by 1,5-2. This applies not only to passenger cars, but also to vans and trucks with low payload. Experts recommend changing the oil every 7-10 000 km, depending on the engine manufacturer’s recommendations, internal combustion engine condition and the type of oil used.

In theory, you can be guided by the alkaline number in oil (TBN). The higher it is, the longer is the mileage, during which the oil will successfully wash the engine and neutralize acid substances and aggressive oxidation products. The optimum TBN value for diesel engines is 11-14.

The second indicator to rely on is the acid number (TAN). The lower it is, the better, i.e. the oil initially has the minimum quantity of substances activating corrosion and friction pair wear in the engine.

For gasoline engines, it is also worth paying attention to this indicator, but for diesel engines it is of particular importance.

Who and Why Change Oil for Winter and Summer
The oil should be changed for the winter and summer in two cases. The first one is when by mileage or moto-hours it fits the season change. The second one is optimal for regions with a sharp changes in climate conditions. The middle zone inhabitants should not take such trouble. All commercially available oils are universal and can be used at any temperature. But if the region has extremely cold winters and hot summers, one should use less viscosity oil in winter, and more in summer.

To summarize

So, let us summarize everything we have talked about in this article:

  1. In the timing of replacement in the first place, we rely on the recommendations of the engine manufacturer and the type of oil used, semi-synthetic oils roll out less than synthetic. The longest intervals are allowed on oil with esters.
  2. In order not to make a mistake and not to over-oil, choose your best method of measuring mileage: moto-hours, kilometrage, calendar period of use, amount of fuel used. To choose a specific method we are guided by the annual mileage, driving style, where the car is used and the quality of fuel it is filled with (the last point is especially important for diesel engines)
  3. Do not forget to assess the appearance of the oil, even if the time of replacement has not yet come. You may run into a fake, the engine may be technically defective – and there are a lot of other reasons why the oil may lose its properties earlier, you may estimate its condition approximately, without a laboratory, visually.
  4. Take into account the load, with which the car is used and its engine power. That is if you like to drive a powerful engine with a big volume, it will be less critical for it than for a small-capacity car. Accordingly, oil aging during aggressive driving will go faster in the second case, on a small-capacity engine.
  5. It is better to use synthetic oils for aggressive and fast driving, and change them more frequently than for those who drive in a measured manner and without increasing revolutions.
  6. When driving in the city with frequent idling in traffic jams, you should be guided by moto-hours or amount of consumed fuel, but not by mileage.
    At a measured drive in the absence of traffic and at an average speed of 50 km / h, feel free to rely on the mileage to choose a method of replacement.
  7. If you do not roll out the manufacturer’s recommended miles per year, then rely on the period of oil use, but not mileage, that is, change it after one year (or the period indicated by the manufacturer, for example, for some engines is half a year),
  8. If your engine manufacturer recommends replacement intervals of more than 10 000, and you want to maintain the specified interval, be sure to choose oils with the tolerance for your engine. Not the conformity, but the tolerance. Such oils are necessarily tested in the laboratory of the automobile manufacturer for all indicators, including the ability to withstand the specified interval.
  9. For diesel engines, pay attention to the alkali and acid numbers in the fresh oil when determining the optimum drain interval.

Related posts:
What Is Multigrade Oil
What Is Engine Oil Volatility
What Is Synthetic Oil

David Muench (Carnes Mechanical)

David Muench (Carnes Mechanical)

Hey, I’m David. I’ve worked in a cars store for 6 years. I write reviews and guides, helping people to choose the most suitable technicals and best product for them. I’m happy to finally share my knowledge of the industry here, on CarnesMechanical.

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