Engine Oil, also known as Motor Oil, is a high-performance lubricant formulated to operate under extreme temperatures, pressures, and mechanical stress inside internal combustion engines. Modern engine oil is an engineered fluid, designed not only for lubrication but also for protection, cooling, and long-term engine reliability.
Beyond reducing friction, engine oil performs five critical functions: lubrication, cooling, cleaning, sealing, and protection against corrosion and oxidation. Proper oil selection and quality directly influence engine life, fuel efficiency, emissions, and maintenance costs.
Primary Functions of Engine Oil
Lubrication
Forms a protective film between moving parts, reducing friction and wear.
Cooling
Transfers heat away from pistons, bearings, and valve components.
Cleaning
Keeps soot, carbon, and contaminants suspended to prevent sludge formation.
Sealing
Seals piston rings against cylinder walls to maintain compression.
Protection
Prevents corrosion, oxidation, and premature engine failure.
Composition of Engine Oil
Engine oil consists of two primary components: base oils and a carefully balanced additive package.
Base Oils (70–90%)
Base oils determine viscosity, volatility, oxidation resistance, and overall
thermal stability.
API Base Oil Groups include:
Group I – Solvent refined (basic performance)
Group II – Hydroprocessed (better purity & stability)
Group III – Severely hydrocracked (synthetic-like performance)
Group IV – PAO (true synthetic)
Group V – Esters and specialty fluids
Additive Package (10–30%)
Additives enhance oil performance under severe engine conditions.
Common additives include anti-wear agents (ZDDP), detergents, dispersants, antioxidants, viscosity index improvers, corrosion inhibitors, friction modifiers, and anti-foam agents.
Viscosity & SAE Grading
Engine oil viscosity is classified by the SAE (Society of Automotive Engineers). A multigrade oil such as SAE 5W-30 provides both cold-start protection and high-temperature stability.
Example:
5W → Cold-temperature (winter) performance
30 → Viscosity at operating temperature (100°C)
Typical SAE grades include:
0W-20 – Extremely cold climates
5W-30 – All-season passenger vehicles
10W-40 – Warm climates
15W-40 – Heavy-duty diesel engines
20W-50 – Older engines and hot regions
Key Performance Characteristics
Viscosity Index (VI)
Indicates viscosity stability across temperature ranges. Higher VI means better
performance at both low and high temperatures.
Thermal & Oxidation Stability
Prevents oil breakdown at elevated temperatures and reduces sludge formation.
Anti-Wear & Extreme Pressure Protection
Protects camshafts, bearings, and piston rings, especially during cold starts.
Detergency & Dispersancy
Keeps contaminants suspended and maintains engine cleanliness.
Fuel Economy Benefits
Lower friction improves engine efficiency and reduces fuel consumption.
Typical Physical & Chemical Properties
Viscosity Index: 120 – 180+
Flash Point: 200 – 250°C
Pour Point: –45°C to –15°C
Total Base Number (TBN): 6 – 15 mg KOH/g
Density: 0.84 – 0.88 g/cm³
Sulphated Ash: 0.6% – 1.5%
API & ACEA Performance Standards
API (American Petroleum Institute)
Petrol engines: SN, SP (latest – LSPI protection)
Diesel engines: CI-4, CK-4
ACEA (European Standards)
A/B – Passenger cars
C – Low-SAPS oils for emission systems
E – Heavy-duty diesel engines
Types of Engine Oil
Mineral Oil – Basic protection, suitable for older engines
Semi-Synthetic Oil – Balanced cost and performance
Fully Synthetic Oil – Superior protection and extended drain intervals
High-Mileage Oil – Reduces leaks and oil consumption
Specialty Oils – Diesel, motorcycle, marine, and generator oils
Engine Oil Selection Guide
Passenger cars: 0W-20 / 5W-30
Diesel trucks: 15W-40 (CI-4 / CK-4)
Motorcycles: 10W-30 / 20W-50
Construction equipment: 15W-40
Generators: SAE 30 / 15W-40
Storage & Handling
Store engine oil in sealed containers, away from moisture and direct sunlight. Maintain cleanliness during handling and transfer to avoid contamination.
Environmental & Emission Compliance
Modern low-SAPS engine oils protect catalytic converters and DPF systems. Extended oil drain intervals reduce waste oil and environmental impact.
Why Engine Oil Quality Matters
Extends engine life
Improves fuel efficiency
Reduces emissions
Prevents costly breakdowns
Ensures reliable performance under all operating conditions