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The Walther equation (ASTM D341): log log(ν+0.7) = A − B × log(T). It produces a straight line on ASTM viscosity-temperature chart paper.

Very accurate for mineral oils from −20°C to 150°C. Less accurate for heavily additized synthetics, near pour point, or near flash point.

VI indicates how much viscosity changes with temperature. Higher = more stable.

• VI < 80: Low (naphthenic base)
• VI 80–100: Normal mineral oil
• VI 100–120: Good (HVI or semi-synthetic)
• VI 120–160: Very good (PAO synthetic)
• VI > 160: Excellent (multigrade, VI-improved)

For most bearings and gears: 13–100 cSt at operating temperature.

• Below 13 cSt: boundary lubrication risk, increased wear
• 13–30 cSt: hydrodynamic bearings, high-speed gears
• 30–100 cSt: general industrial gears and bearings
• Above 100 cSt: may cause churning losses, high startup torque

Kinematic viscosity: 1 cSt = 1 mm²/s exactly.

Dynamic viscosity: η = ν × ρ. Units: mPa·s = cSt × g/cm³. For typical oil at 15°C: ρ ≈ 0.87 g/cm³, so 100 cSt = 87 mPa·s.

SUS (Saybolt): roughly SUS ≈ 4.6 × cSt (for cSt > 50).

Pour point is the lowest temperature at which oil still flows. Below pour point, wax crystallization makes the oil too thick to pump. It matters for cold-start conditions, outdoor equipment, and Arctic applications.

Mineral oils: typically −15 to −30°C. PAO synthetics: −40 to −60°C.