What is the difference between ISO VG and SAE?

ISO VG (Viscosity Grade) classifies industrial lubricants by kinematic viscosity at 40°C. SAE grades are used for automotive oils: SAE crankcase grades (e.g. 10W-30) are based on viscosity at 100°C, while SAE gear grades (e.g. 75W-90) use a different scale. The systems measure at different temperatures and serve different markets.

Why are there different viscosity scales?

Different industries developed their own standards independently. ISO VG (ISO 3448) is used for industrial lubricants, SAE J300 for automotive engine oils, SAE J306 for automotive gear oils, and AGMA 9005 for industrial gears. Each standard was designed for its specific application and has different test conditions.

AGMA (American Gear Manufacturers Association) defines lubricant grades for industrial gearing per AGMA 9005. AGMA grades 1-8A correspond roughly to ISO VG 46 through VG 1000. The AGMA system is commonly used in North America for specifying gear lubricants.

What is the difference between cSt and cP?

cSt (centistokes, mm²/s) is kinematic viscosity — it measures flow under gravity. cP (centipoise, mPa·s) is dynamic viscosity — it measures resistance to flow under applied force. They are related by: cP = cSt × density (g/cm³). For most lubricating oils with density ~0.87 g/cm³, cP ≈ 0.87 × cSt.

How to read multigrade SAE oil?

Multigrade SAE oils like 10W-30 have two numbers: the 'W' (Winter) number indicates cold-start viscosity, and the second number indicates viscosity at operating temperature (100°C). A 10W-30 oil flows like a 10W at cold temperatures but provides the protection of a 30-grade at operating temperature. Multigrade oils use viscosity index improvers to achieve this behavior.

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黏度轉換器

ISO VG、SAE 曲軸箱、SAE 齒輪與 AGMA 黏度等級的交叉參考。亦可轉換 cSt、cP 與 SUS 單位。

ISO VG SAE J300 / J306 AGMA 9005 cSt ↔ cP ↔ SUS

結果

ISO VG 等級

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SAE 曲軸箱等級

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SAE 齒輪等級

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AGMA 等級

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黏度範圍

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動力黏度

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SUS（Saybolt Universal）

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ISO VG / SAE / AGMA 交叉參考

黏度分級系統之間的近似等效關係。這些並非精確轉換——各系統採用不同的測量條件。

ISO VG	cSt 範圍（40 °C）	SAE 曲軸箱	SAE 齒輪	AGMA

單位轉換公式

動力黏度 (cP) = 運動黏度 (cSt) × 密度 (g/cm³)

其中：1 cP = 1 mPa·s，且 1 cSt = 1 mm²/s。油密度通常為 0.85–0.90 g/cm³ (850–900 kg/m³)；若需精確換算，請使用與運動黏度相同溫度下的密度（40 °C 的密度約比 15 °C 低 1.5 %）。

SUS ≈ cSt × 4.635 （近似值，適用於 cSt > 50）

Saybolt Universal Seconds (SUS) 近似公式適用於黏度高於約 50 cSt 的情況。對於較低黏度，需使用更複雜的公式，但上述公式對大多數工業用油可提供合理的結果。

關於分級系統

ISO VG （ISO 3448）——工業潤滑油；18 個等級 VG 2 至 VG 1500；40 °C 時的運動黏度 ± 10%。
SAE 曲軸箱 （SAE J300）——汽車引擎油；等級如 10W、20、30、40、50；於 100 °C 測量。
SAE 齒輪 （SAE J306）——汽車齒輪油；等級如 75W、80W-90、140、250；於 100 °C 測量。
AGMA （AGMA 9005）——工業齒輪潤滑油；等級 1 至 8A；與 ISO VG 對齊。

⚠️ 重要： 交叉參考僅供近似。ISO VG 於 40 °C 測量，而 SAE 於 100 °C 測量。請務必參考特定油品產品資料表，以確認您在操作溫度下的精確黏度值。