Viscosity index improvers – additives for stable lubricating performance at all temperatures
Viscosity index improvers (VII) are polymer-based additives that are used in engine oils, transmission oils and hydraulic oils to minimize the change in viscosity with temperature differences. They ensure that an oil remains thin when cold, but does not lose too much viscosity when hot, i.e. provides stable lubrication – from cold start to full load.
Why are viscosity index improvers crucial for modern oils?
Modern engines and machines operate under very different conditions – from winter starts at -20 °C to hot summer operation at 120 °C oil temperature. VI improvers are used to ensure that the oil functions reliably in both cases. They uncoil when warm and thus increase the viscosity without impairing the cold start behavior.
BASINOL multigrade oils use precisely matched viscosity index improvers to ensure a stable lubricating film over the entire temperature range – for maximum wear protection and optimum efficiency.
How do viscosity index improvers work in oil?
These additives consist of long-chain, temperature-reactive polymers. At low temperatures, they are contracted and hardly affect the flow behavior – so the oil remains pumpable. As the temperature rises, they expand, which increases the viscosity and prevents the oil from becoming too thin. The result: a stable oil film when hot and good flowability when cold.
Are there differences between viscosity classes with and without VII?
Yes. Monograde oils (e.g. SAE 30) usually do not require a viscosity index improver – they are only suitable for constant temperatures. Multigrade oils (e.g. 10W-40, 5W-30) contain VII in order to cover a wide temperature range. The wider the viscosity range, the higher the proportion of viscosity index improvers in the oil.
Can VI improvers degrade or wear out in the oil?
Yes – under heavy mechanical stress (shear) they can break down, which can lead to a loss of viscosity. High-quality lubricants such as those from BASINOL therefore use shear-stable viscosity index improvers, which function reliably even under continuous load and keep the viscosity stable over the entire change interval.
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