Viscosity Index (VI) is a standardized, unitless number that tells you how much an oil’s viscosity changes with temperature. It’s a critical characteristic that helps users understand how an oil will perform during a cold start, at normal operating temperatures, and in high-heat environments.
This is a key point: VI does not describe an oil’s actual viscosity, like its 10W-30 or 5W-40 grade. Instead, it describes the rate of viscosity change as the oil heats up or cools down. Two oils with the same 10W-30 grade can have vastly different VIs. The one with a higher VI will offer more consistent protection across a wider temperature range.
An oil with a stable viscosity across temperature swings is critical for a modern engine’s health and performance.
Modern engines—especially turbocharged, high-performance, and direct-injection units—demand oils with a high VI to maintain peak performance and longevity.
Viscosity Index is calculated using ASTM D2270, a standardized method from the American Society for Testing and Materials.
The process works by comparing the test oil to two reference oils: a naphthenic oil with a VI of 0 and a paraffinic oil with a VI of 100.
High-performance synthetics or oils with custom additive blends may exceed a VI of 200.
As engine temperature rises, oil thins. The rate at which it thins is where VI matters.
An oil with a low VI might lose its protective film strength too quickly as it heats up, leaving metal surfaces vulnerable. An oil with a high VI, however, maintains its protective qualities across a wide temperature range, making it more versatile for varied driving conditions, hot climates, or heavy-duty use.
Example:
The type of base oil is one of the biggest factors in an oil’s VI.
High-quality synthetic base oils (especially Group IV) have a naturally high VI, which means they don’t need to rely as heavily on additives to maintain their viscosity. This leads to better long-term stability and performance.
To meet the viscosity targets of a multigrade oil (like 5W-30), oil formulators often use VI improver additives. These are long-chain polymer molecules that expand at higher temperatures, counteracting the oil’s natural tendency to thin.
Since high-quality synthetic oils have a naturally high VI, they require fewer VI improvers. This results in superior shear stability, longer drain intervals, and more consistent protection over the life of the oil.
Choose high VI oils for modern engines, severe service, or variable climates.
Conventional oils have a naturally lower VI and rely heavily on VI improver additives.
Synthetic oils (especially PAO-based ones) have a higher inherent VI, better oxidation resistance, and lower volatility.
This is why synthetic oils perform better at both hot and cold extremes, are specified for turbocharged and high-RPM engines, and can support longer drain intervals. A high VI is one of the defining advantages of synthetic oil technology.
A VI above 140 is typically considered high. Premium synthetic oils can often exceed 180–200.
Indirectly, yes. Oils with a higher VI tend to be more stable and resist breakdown better, which supports extended drain intervals when combined with a proper additive package.
No. VI is about how viscosity changes with temperature, not the grade itself. Two 10W-30 oils can have vastly different VIs.
VI is calculated using ASTM D2270, the industry’s accepted method for measuring and comparing an oil’s temperature-dependent viscosity changes.