Viscosity index – measure of the temperature stability of a lubricating oil
The viscosity index (VI) indicates how much the viscosity of an oil changes when the temperature changes. The higher the VI value, the more constant the lubricity of the oil remains – regardless of whether the engine is cold-started or running hot. The viscosity index is therefore an important quality indicator for engine oils, transmission oils and hydraulic oils, especially under changing temperature conditions.
What does the viscosity index tell you about your oil?
A lubricant with a high VI is particularly stable at low and high temperatures. This is crucial for applications in which a wide temperature range must be reliably covered – for example in vehicle operation, in construction machinery or industrial systems in outdoor areas.
BASINOL lubricants are based on high-quality base oils and special VI improvers to ensure a stable lubricating film even under extreme conditions.
What is a “good” viscosity index?
A VI below 100 is considered low and shows a strong change in viscosity when the temperature changes – typical for classic mineral oils.
VI values of 120-160 stand for modern multigrade oils with good temperature stability, such as SAE 5W-30.
High-VI oils above 160 – such as those based on PAO or ester – offer excellent lubricating performance at temperature peaks, such as those found in motorsport or high-performance machines.
How is the viscosity index calculated?
The VI results from the kinematic viscosity at 40 °C and 100 °C, in comparison to two reference oils with defined behavior. A high VI means: lower viscosity change per temperature difference – the oil remains in its optimum working range. The formula has been standardized by ASTM (D2270).
How does the viscosity index affect engine or system operation?
A stable viscosity curve protects against:
– Lubricant film breakage in heat
– Cold start problems due to excessively viscous oil
– Increased wear due to viscosity fluctuations
This is why high-quality lubricants such as BASINOL oils rely on base oils with a high VI and high-performance VI improvers to ensure constant lubrication over the entire temperature range.
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