Viscosity index: the number that matters more than the grade on the label

When you choose a 10W-40, you are choosing an oil that passes a viscosity test at 100°C. What you are not choosing is how well that oil holds its viscosity when your engine is actually hot. Two oils both labelled 10W-40 can behave very differently at 110°C — the kind of temperature your engine reaches during a slow commute in congested traffic. The number that describes this difference is the Viscosity Index.

What Viscosity Index measures

All oils thin as temperature rises. The Viscosity Index — abbreviated VI — is a dimensionless number that describes how much an oil thins as temperature increases. A higher VI means less thinning: the oil holds its viscosity more consistently across a temperature range. A lower VI means the oil changes viscosity more dramatically with temperature.

A 10W-40 label tells you the oil passes a viscosity test at 100°C. It says nothing about viscosity at 110°C, 115°C, or 120°C — the temperatures your engine oil may sustain during stop-start urban riding in SEA.

VI by base oil group

VI varies significantly by base oil type. Naphthenic and paraffinic mineral oils (Group I): VI 60–100. Group II hydrotreated mineral oils: VI 80–120. Group III hydrocracked “synthetic” mineral oils: VI ≥120 (legally permitted to be called synthetic since 1999). Group IV PAO synthetic: VI typically 140–175 depending on grade.

An oil labelled “fully synthetic” using Group III base stock might have a VI of 125. An oil using PAO (Group IV) base stock might have a VI of 155. Both pass the 10W-40 test at 100°C. At 110°C, they are not the same oil.

Why VI matters more in SEA than in cooler climates

Engine oil temperature in a small motorcycle engine during SEA stop-start commuting often reaches and sustains 100–120°C. A Group II-based 10W-40 (VI ~100) may be operating at the outer edge of its viscosity stability. A Group III-based 10W-40 (VI ~125) holds its grade better under the same conditions. A PAO-based 10W-40 (VI ~155) holds its grade significantly better — closer to its 100°C specification than either alternative at real operating temperature.

In a country with 15°C ambient and predominantly highway riding, the gap between VI 100 and VI 155 narrows. In a country with 32°C ambient, congested urban roads, and air-cooled engines — the gap is real and measurable in protection and oil life.

Does the label tell you VI?

Sometimes. VI is a standard property that should appear on any product data sheet (PDS). It may or may not appear on the bottle. If the PDS states VI above 140, the oil is using Group IV PAO or high-quality Group V base stocks. If VI is unstated on the PDS, it is typically because it is not a selling point.

VI is not the same as HTHS — but both matter

VI and HTHS viscosity are related but distinct properties. VI describes how viscosity changes with temperature under no shear. HTHS describes viscosity under shear stress at 150°C. An oil can have a high VI but still thin under shear if it relies on polymer additives to achieve that VI. PAO oils tend to score well on both: high VI from molecular structure, and high inherent shear resistance from the same molecular uniformity.

Key takeaways

• VI measures how much an oil's viscosity changes with temperature. Higher VI = more stable viscosity across the operating range.
• Group I/II mineral oils: VI 60–120. Group III “synthetic”: VI ≥120. PAO (Group IV): VI typically 140–175.
• Two 10W-40 oils can have very different protection at 110°C if their VI values differ — the grade label does not reveal this.
• In SEA stop-start conditions, sustained high oil temperatures make VI more consequential than in cooler climates.
• Look for VI on the product data sheet. If a brand does not publish a PDS, or omits VI from it, that is a signal.
• VI and HTHS are separate properties. PAO scores well on both. Polymer-boosted Group III oils may score well on VI but thin under shear.