Viscosity grades of lubricant oil define the oil’s resistance to flow at specific temperatures and are critical for ensuring proper lubrication, protection, and efficiency of engines and machinery. Viscosity directly determines how effectively the oil forms a lubricating film between moving parts under varying operating conditions.
Lubricant viscosity grades are primarily classified by SAE (Society of Automotive Engineers) standards for automotive oils and ISO VG (International Organization for Standardization – Viscosity Grade) standards for industrial lubricants.
Selecting the correct viscosity grade ensures: easy oil flow during cold starts, adequate film strength at high operating temperatures, and reduced friction, wear, and energy loss.
Types of Viscosity Grading Systems
1. SAE Viscosity Grades (Automotive Oils)
SAE grades describe oil performance at both low and high temperatures.
Single-Grade Oils
SAE 30, SAE 40, SAE 50 oils perform optimally within a limited temperature range
and are commonly used in older engines or in environments with stable operating
temperatures.
Multi-Grade Oils
Examples include SAE 5W-30, 10W-40, and 15W-50.
The letter “W” stands for Winter.
In a multi-grade oil: the first number indicates cold-temperature performance, while the second number represents viscosity at operating temperature.
Example – SAE 10W-30
Flows like an SAE 10 oil at low temperatures and protects like an SAE 30 oil at
normal engine operating temperature.
2. ISO VG Grades (Industrial Lubricant Oils)
ISO viscosity grades classify oils based on their kinematic viscosity at 40°C,
measured in centistokes (cSt).
Common ISO VG grades include: ISO VG 22 for very light oils, ISO VG 32 and 46 for hydraulic systems and compressors, ISO VG 68 and 100 for gearboxes and bearings, and ISO VG 150 or 220 for heavy-load industrial gears.
Each ISO VG grade allows a ±10% viscosity variation from its nominal value.
Key Characteristics of Viscosity Grades
Flow Resistance
Low-viscosity oils flow easily and reduce drag,
while high-viscosity oils provide thicker protective films.
The correct balance prevents metal-to-metal contact.
Temperature Sensitivity
Oil thickens at low temperatures and thins at high temperatures.
High-quality oils maintain stable viscosity, preventing cold-start wear and
high-temperature oil thinning.
Viscosity Index (VI)
The viscosity index indicates how much an oil’s viscosity changes with temperature.
A higher VI means better viscosity stability.
Typical VI ranges include: 90–105 for mineral oils, 110–130 for semi-synthetic oils, and 130–170+ for fully synthetic oils.
Lubrication & Wear Protection
Correct viscosity ensures a strong oil film, reduced friction and heat generation,
and extended equipment life.
Oil that is too thin increases wear, while oil that is too thick causes power loss
and overheating.
Fuel Efficiency & Energy Savings
Lower viscosity oils reduce internal resistance, improving fuel economy and
mechanical efficiency, especially in modern engines and energy-efficient machinery.
Refining & Formulation of Viscosity-Grade Oils
Base Oil Refining
Solvent refining removes aromatics and impurities,
hydrocracking produces stable, high-purity base oils,
and severe hydroprocessing improves oxidation resistance and viscosity index.
Dewaxing Process
Wax molecules are removed to enhance low-temperature flow,
which is critical for winter and low-pour-point grades.
Additive Technology
Additives are blended to fine-tune viscosity behavior and protection,
including viscosity index improvers, anti-wear agents, antioxidants,
detergents, dispersants, and anti-foam agents.
Multi-Grade Stability
In multi-grade oils, polymer-based viscosity index improvers expand at high
temperatures to maintain oil thickness under load while resisting shear breakdown.
Typical Viscosity Ranges
SAE 5W – Cold-temperature cranking compliant
SAE 30 – Approximately 9.3 to 12.5 cSt at 100°C
ISO VG 32 – 28.8 to 35.2 cSt at 40°C
ISO VG 46 – 41.4 to 50.6 cSt at 40°C
ISO VG 68 – 61.2 to 74.8 cSt at 40°C
Applications Based on Viscosity Grade
Automotive & Transportation
Passenger vehicles, commercial trucks, marine engines, and construction equipment.
Industrial Applications
Hydraulic systems, compressors, turbines, gearboxes, bearings,
and circulating oil systems.
Specialized Equipment
Low-viscosity oils for high-speed machinery,
high-viscosity oils for heavy-load equipment,
and special grades for extreme temperature environments.
Importance of Selecting the Correct Viscosity Grade
Using the correct viscosity grade reduces mechanical wear, improves fuel and energy efficiency, prevents overheating and oil breakdown, extends oil drain intervals, and ensures manufacturer warranty compliance.
Incorrect viscosity selection can result in excessive wear, sludge formation, poor performance, and higher operating costs.