Temperature change and viscosity index

Oil softens when it warms up and hardens when it cools down. You all know this. Therefore, the basic rule is to choose the SAE viscosity specified in the instruction manual to avoid the inconvenience of oil that is too hard or too soft. However, just stating the basics is tasteless, so let's go one or two steps further. Kinematic viscosity and viscosity indexKinematic viscosity is the viscosity, or hardness, of a liquid. Kinematic viscosity changes depending on temperature, but in general, kinematic viscosity at 40°C and 100°C are often indicated. The reason for this is the viscosity index. The viscosity index indicates the degree of change in viscosity due to changes in temperature, and its standard is the change in kinematic viscosity at 40°C and 100°C. Since the viscosity index is one indicator of oil performance, the kinematic viscosity at 40°C and 100°C is important. Another reason to look at kinematic viscosity. Even if the SAE viscosity says 0W-30, there are hard and soft oils within that range. The kinematic viscosity can be used as a reference when comparing them and finding the most suitable one for your car. ● Roughly speaking, what influences the viscosity index is the degree of refinement of the base oil and the viscosity index improver. Generally, the larger the molecule, the higher the viscosity. If there is a mixture of large and small molecules. The difference in viscosity at low and high temperatures will be large, i.e., the oil will have a low viscosity index. This is the reason why mineral oil has a low viscosity index. Gr. III VHVI, which is called a synthetic oil derived from mineral oil, is a base oil with a high viscosity index because it is refined from mineral oil to have the desired molecular weight. If there is a substance that flows smoothly like a small molecule at low temperatures and sticks like a high molecule at high temperatures, we can increase the viscosity index. There is such a convenient substance. This is called a viscosity index improver. General engine oils are mixed with this to increase the viscosity index and maintain performance over a wide temperature range. High-temperature high-shear viscosity is also called HTHSV or HTHS viscosity for short because it is long. In engine oils, viscosity is generally measured at a high temperature of 150°C and the number is evaluated. What this indicates is an indicator of the oil's toughness (high temperature resistance). Earlier, we talked about viscosity index improvers. This is only an additive to increase the viscosity index, not the oil (base oil) itself. Therefore, it cannot withstand high temperatures and high pressures, and its viscosity will decrease. Although the viscosity decreases, the resistance is reduced, but the anti-wear effect of the parts also decreases. Therefore, HTHSV is used as a reference to determine the high temperature resistance of the oil itself, which is not affected by viscosity index improvers. However, it cannot be said that a high HTHSV is a good oil, because the oil itself has high viscosity when the HTHSV is high, so there is an aspect that resistance is high. In a car where the oil temperature is kept low or at an appropriate temperature, oil with a low HTHSV and a high viscosity index will have less friction loss and run more comfortably. On the other hand, for high-output vehicles and air-cooled engines, where oil temperature tends to be high, it is better to choose oil with a high HTHSV to maintain the protection performance of parts even at high temperatures.

Click here to purchase viscosity index improvers and viscosity modifiers.
The numbers in the product names indicate the kinematic viscosity at around 100℃.

Polyalphaolefin Series
A special base oil that is tough and has low resistance.




Alkyl Naphthalenes Series
By adding alkyl naphthalene, the oil deterioration is suppressed and the oil film is strengthened.
In addition, it is a special base oil that has an oil film strengthening function.






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