What is the Composition of Engine Oil (Automotive Engine Lubricant)?

The composition of engine oil isn’t as complicated as it seems—it primarily consists of two components: base oil and additives. Think of it like the fusion of Goku and Trunks from Dragon Ball Z: together, they form something greater than the sum of its parts.

Additives: Enhancing Core Functions

Additives are designed to strengthen engine oil’s key functions: lubrication, cleaning, cooling, and oxidation resistance. Their exact composition will be detailed later, but for now, note that while additives are critical, they are not the majority component of oil.

Base Oil: The Foundation (85% of the Bottle)

Base oil makes up roughly 85% of engine oil by weight, so let’s start here.

How is Base Oil Classified? Mineral vs. Synthetic

In 1919, the American Petroleum Institute (API)—founded by nearly 400 oil exploration, transportation, refining, and sales companies—set out to standardize base oils. Today, base oils are categorized into five groups:

A Note on Shell’s XHVI Technology: Shell’s patented XHVI (Hydrogenated Isomerization) process creates oils with performance comparable to Group IV PAO synthetic oils. For this reason, Shell labels its XHVI oils as "synthetic" (Group III), while other brands use terms like "fully synthetic" or "100% synthetic" for Group IV/V oils.

Strictly Speaking: Fully synthetic oil refers to products made exclusively from Group IV (PAO) or Group V (ester-based) base oils.

Warning for Group V Users: Ester-based oils (Group V) offer the best lubrication but are unstable in water, corrode seals/coatings, and are costly. Store them tightly sealed.

What is 5W-40? Understanding Viscosity

The history of engine oil standards dates to the early 20th century, when the industry lacked uniformity. In 1911, the SAE (Society of Automotive Engineers) began developing guidelines, culminating in SAE J300—the standard that enables us to select oils for diverse vehicles, climates, and regions today.

The Meaning of "W" (Winter)

Early SAE standards only described high-temperature viscosity. Later, the "W" (Winter) was added to indicate low-temperature performance, creating the familiar "5W-40" format.

• "5W": Suitable for use in environments as cold as -35°C (the "W" stands for Winter). Lower "W" numbers mean better low-temperature fluidity.
• "40": Indicates high-temperature viscosity. Higher numbers mean thicker oil at high temps.

Two Key Notes:

1. SAE grades are general guidelines, not absolute measures of an oil’s full viscosity range.
2. Always follow your vehicle’s owner’s manual for recommended SAE viscosity.

API Standards: Beyond Viscosity (SM, SN, etc.)

The API (American Petroleum Institute) sets standards focused on performance metrics like acceleration, noise reduction, longevity, and TBN (Total Base Number).

Gasoline Engine Ratings (SJ, SL, SM, SN)

• "S" denotes gasoline engine oil; subsequent letters (J, L, M, N) indicate quality tiers.
• Key differentiators: Zinc and phosphorus content (restricted to reduce heavy metal emissions and PM2.5 pollution).

Recommendation: While older tiers (e.g., SL) still work, opt for SN if you prioritize environmental protection.

Global Standards: ILSAC, ACEA, and Manufacturer-Specific Specs

Beyond SAE and API, other organizations and automakers enforce their own standards. Key examples include:

ILSAC (International Lubricant Standardization and Approval Committee)

Aligned with API, ILSAC focuses on fuel efficiency and emission compliance. Common grades: GF-5, GF-6.

ACEA (European Automobile Manufacturers’ Association)

Widely adopted by European automakers, ACEA standards (e.g., A3/B4, C3) emphasize high-performance lubrication.

Manufacturer-Specific Standards (Examples)

• Volkswagen: VW502.00/505.00 (0W-40/5W-40), VW502.00/505.01 (0W-40/5W-40), VW504.00/507.00 (5W-30). All require A3/B4 ACEA compliance.
• BMW: Longlife-01 (ACEA A3/B4). Only ACEA A3/B4 synthetic oils (5W-30, 0W-40, 5W-40) are approved.
• Mercedes-Benz: MB 229.3/229.5 (ACEA A3/B4). Newer specs include 5W-30; diesel engines use C3.
• General Motors: Dexos1 (gasoline) and Dexos2 (gas/diesel). Dexos1 offers better protection than standard SN/GF-5 but slightly lower fuel efficiency.
• Toyota/Lexus: Most require ILSAC GF-5 (explaining Toyota’s fuel efficiency). Always check the owner’s manual for viscosity specifics.

What Do These Standards Measure? Key Performance Metrics

Standards exist to quantify critical performance traits. Here are the most important:

1. 100°C Kinematic Viscosity (100CST)

The primary indicator of an oil’s "true" viscosity. Engines have components operating at varying temperatures, so oil must maintain optimal lubrication across conditions. SAE grades 5.6–26 cSt at 100°C.

• Higher 100CST: Better for high-temp, high-stress engines (e.g., turbocharged engines, which run up to 400°C).
• Lower 100CST: Preferred for low-stress engines (e.g., Japanese naturally aspirated engines).

Note: 100CST is context-dependent—match it to your engine’s needs (per the owner’s manual).

2. High-Temperature High-Shear (HTHS) Viscosity

Measures lubrication at high temps (e.g., crankshaft, camshaft contact points). HTHS >3.5 cP is critical for German engines recommending 30-grade oil.

3. Total Base Number (TBN)

Indicates the oil’s cleaning power. TBN measures alkaline additives that neutralize acidic byproducts of combustion. Higher TBN = longer oil life.

4. Pour Point (Low-Temperature Fluidity)

Critical for cold climates. The pour point is the lowest temperature at which oil flows. A lower pour point ensures reliable engine startup in winter.

5. Other Metrics

• Viscosity Index (VI): Measures how much viscosity changes with temperature. Higher VI = better performance across temps.
• Flash Point: The temperature at which oil emits flammable vapors (higher = safer).

How Do Additives Deliver These Performance Gains?

While base oils are refined, additives are the unsung heroes that meet strict standards. Key additives include:

• Detergents: Metal salts that remove sludge and varnish from high-temp engine parts.
• Dispersants: Prevent low-temp sludge from clogging filters, valves, and piston rings.
• Antioxidants: Slow oxidation (which darkens oil and increases viscosity).
• Rust/Corrosion Inhibitors: Protect metal surfaces from water, salt, and acid.
• Viscosity Index Improvers: Enable multi-grade oils (e.g., 5W-30) to maintain viscosity across temps.
• Pour Point Depressants: Prevent wax crystals from thickening oil in cold weather.
• Anti-Wear Additives: Form protective films to prevent metal-to-metal friction under high pressure.
• Anti-Foaming Agents: Reduce foam (which reduces lubrication and accelerates oxidation).
• Dyes: For identification (e.g., red for ATF) or marketing (e.g., blue/light red for "refined" base oils).

30-Second Oil Selection Guide: Just Follow the Manual

After all this detail, here’s the simplest rule: Open your owner’s manual, flip to the "oil" section, and follow the recommended SAE viscosity and manufacturer-specific standard (e.g., VW502.00, BMW Longlife-01).

This guarantees optimal performance, engine protection, and warranty compliance. No need for complex calculations—your manual is your best guide.