Viscosity is the property everyone talks about when selecting a lubricant. SAE 5W-30 is thinner than SAE 15W-40. ISO VG 46 is heavier than ISO VG 32. These grades are useful shorthand, but they describe a single number — the kinematic viscosity at a reference temperature — and a lubricant's performance under real operating conditions depends on much more than that single number.

What Viscosity Grade Actually Measures

ISO VG grades measure kinematic viscosity at 40°C in centistokes (cSt). SAE grades for engine oils measure kinematic viscosity at 100°C. These are standardised test conditions. A lubricant graded ISO VG 46 has a kinematic viscosity between 41.4 and 50.6 cSt at 40°C — that is a band, not a point.

The measurement is an average property of a fluid made up of millions of individual molecules. Those molecules are not identical. In a synthetic lubricant base stock, they are polymer chains of varying lengths. In a mineral oil, they are a complex mixture of hydrocarbon structures from crude oil distillation. The average viscosity may be the same, but the distribution of molecular sizes — the variance — differs significantly between formulations, and that variance matters under operating conditions.

Polyalphaolefins and Molecular Weight Distribution

Polyalphaolefins (PAOs) are the most common synthetic base stock in premium industrial lubricants. They are synthesised by polymerising alpha-olefin monomers — typically 1-decene or 1-dodecene — to produce chains of controlled length. PAO 4 (approximately 4 cSt at 100°C) contains relatively short chains. PAO 40 (40 cSt at 100°C) contains much longer chains.

The manufacturing process for PAOs produces a narrower molecular weight distribution (MWD) than mineral oil refining. This is one of the primary reasons PAOs outperform mineral oils in demanding applications: a narrower MWD means more consistent behaviour across operating temperatures and shear conditions. Mineral oils, even when highly refined, have a broader MWD because they derive from the complex hydrocarbon mixture in crude oil.

Viscosity Index and Temperature Sensitivity

Viscosity Index (VI) measures how much a fluid's viscosity changes with temperature. A high VI means viscosity stays relatively stable across temperatures. A low VI means the fluid thins dramatically when hot and thickens excessively when cold.

The MWD directly influences VI. Fluids with narrow, uniform molecular weight distributions tend to have higher VI. PAOs typically achieve VI values of 130–160 without additives. Conventional mineral oils achieve 95–100. The practical difference: a PAO-based gear oil in an outdoor industrial application performs consistently from −20°C morning startup to 100°C operating temperature. A mineral oil in the same application may struggle to pump at startup and run thinner than ideal under full load.

Shear Stability

Under high shear conditions — in the narrow gap between gear teeth, in a hydraulic pump, in a journal bearing — long polymer chains in the lubricant can be physically broken. This is called mechanical shear degradation. The chains snap, reducing average molecular weight and therefore viscosity. A lubricant that enters service as VG 46 may behave as VG 32 after extended operation in a high-shear application.

Shear stability is quantified by the Permanent Shear Stability Index (PSSI) — lower is better, meaning less viscosity loss under shear. PAOs generally have better inherent shear stability than mineral oils of equivalent viscosity because their molecular architecture is more resistant to chain scission.

Viscosity index improvers (VII) — polymer additives added to mineral oils to boost their VI — are particularly susceptible to shear degradation because they are specifically designed as long-chain polymers. An oil formulated to VI 130 using a mineral base plus VII can degrade to VI 110 in service. A PAO at VI 150 loses much less because the high VI is intrinsic to the base stock, not additive-dependent.

Practical Implications for Equipment Specification

When specifying a lubricant for a precision application — machine tool spindles, hydraulic servo systems, turbine bearings — the viscosity grade alone is not sufficient information. The relevant questions are: what is the viscosity index, what is the MWD (if the data sheet specifies it), and what is the shear stability index?

For general industrial applications — electric motor bearings, gearboxes in moderate environments — the viscosity grade and change interval are typically sufficient. For demanding applications with wide temperature ranges, high shear loads, or extended oil change intervals, the distribution characteristics of the base stock matter as much as the viscosity grade itself.

The Density Calculator handles material property calculations for engineering applications requiring precise fluid characterisation.