The manufacturer recommends oil 0W-20, but car service pours a thicker one, with an index of 5W-40. Could this adversely affect the condition of the engine?
The viscosity-temperature characteristics of engine oil significantly affect almost all the main indicators of the engine. Power, torque, efficiency, resource – all this is calculated by the developers of the engine for certain oil viscosity. It is classified according to the SAE system (English society of automotive engineers). This classification stipulates the maximum low-temperature viscosity, as well as the viscosity range at 100 ºС / 212 ºF. But in order to understand which oil to choose, you first need to remember what the oil designations hide.
Symbols and undermeaning
They are very arbitrary. The first digit indicates the minimum temperature that the oil is designed for. If, for example, there is zero in front, then cranking is guaranteed at temperatures up to –35 ºС / –31 ºF, and oil pumpability – up to –40 ºС / –40 ºF. More precisely, the oil producer warrants that the viscosity of the product will not exceed the values determined by the SAE classification at the indicated temperatures.
The number after the hyphen is responsible for high temperatures: According to Mechanic FAQ, it indicates the acceptable range for changing the viscosity of oils for yz250f 100ºС / 212 ºF. For example, for the “twenty” the manufacturer promises a fork from 5.6 to 9.3 CST, and for the “forty” – from 12.6 to 16.3 CST. In addition, the same number characterizes the minimum viscosity at 150 ºС / 302 ºF .
Which viscosity is better?
In the cold, everything is clear: the starter will not crank the motor with too viscous oil, and the pump will not be able to pump oil. The smaller the first number in the designation, the less engine wear at starting. This parameter does not affect the operation of a warm engine.
At high temperatures, it is more complicated. It would seem that the higher the viscosity, the better. But this is not so. If you fill in the engine of an ordinary passenger car with the “sixty” oil, not intended for it at all, you will most likely not only lose power but also ruin the engine. But why? After all, viscous oil should better protect parts from wearing out. The higher the viscosity, the thicker the oil layer in the bearings and under the piston seals and, accordingly, the lower the wear rate.
However, there is the flip side to the coin of low thermal conductivity of the oil. After all, the thicker the oil layer, the worse the heat is removed from the piston, which begins to overheat and expand. Friction also grows, and it may lead to self-locking.
Note that the second number “works” not only at three-digit temperatures, but also during engine warm-up. The higher the viscosity, the greater the friction loss. Viscosity depends on temperature. When conducting research, it was found that at room temperature the difference in viscosity between the “thirty” and “fifty” is almost double. And therefore, fuel consumption for more viscous oil during heating-up will be higher.
Now, the main question: what kind of oil is needed exactly for my engine? Unfortunately, modern research has shown that when choosing the right oil for a particular engine, SAE compliance alone is not enough. A more precise “mastering” is needed, depending on both on the design of the engine and on the conditions of its operation and the degree of wear.
The correct answer has long been known. Top up your engine only with the oil recommended by the car manufacturer! Because the manufacturer, as a rule, tries to please the maximum number of consumers, regardless of the operating conditions of the car and its age. The quality group must be respected: if it says SN – that means you cannot pour anything from the SM group. But you can play with viscosity, as it is a part of the permissible. For example, for operation at low temperatures, the second number in the designation may be slightly less than the recommended instruction — say, 30 instead of 40. This will help to slightly reduce fuel consumption, because in winter the oil warms up longer than in summer, and so the consumption of viscous oil will be higher.
The same applies to cars that live mostly in urban environments. If the engine runs mostly at moderate speeds, then the second number in the oil designation may be slightly lower than the recommended one for a car that often travels on highways. The reason is the interrelation between the thickness of the oil film, temperature, and friction. Employees of specialized laboratories claim that there is an optimal viscosity that reduces mechanical losses for each engine and its operating mode.
A bit of amateur activity
However, one question remains. How different are the performance characteristics of oils with the same viscosity, but from different manufacturers? This is a question that cannot be answered without laboratory research. But many automobile fans, especially in cold regions, make their own measurements, constructing home-made devices for the comparative determination of viscosity. The most intuitive design seems to be a ball viscometer.
The falling time of a steel ball in a glass tube (the diameter of the ball is only slightly less than the diameter of the tube) filled with oil indirectly indicates the viscosity of the product. By the way, a similar principle is used in professional devices, for example, in the Geppler viscometer. It will not be possible to measure exactly how many poises or centistokes are in oil, but it allows you to visually compare the viscosity of several oils under identical conditions.
The tube in which the steel ball quickly reaches the bottom has a lower viscosity. And if you get a can of oil from an unknown manufacturer, then organizing the simplest tests will be quite simple. Say, in one test tube we fill the tested oil, in the other a novice, then we put all this in the freezer (or even in a snowdrift), and after exposure, we turn the tubes over and watch for the smooth lowering of the balls. Where the ball drops more slowly, there is a higher viscosity.
We advise all lovers of experiments to build something like this before pouring an unknown fluid into your engine. The clarity of the experiment is guaranteed.
But the car is not worth experimenting with. Anyways, we strongly recommend that you listen to the recommendations of the engine manufacturer, and not the oilmen.