During cold weather, many drivers notice that engine oil thickens, making the engine crank slowly or even fail to start. This is a common phenomenon, but the cause is not only temperature. It is closely related to the physical and chemical properties of the oil itself. The key factor behind this change is the Viscosity Index (VI) — a critical measure that determines how oil behaves across different temperatures. Understanding VI helps users choose the right lubricant to ensure consistent performance and long-term engine protection.
WHY ENGINE OIL THICKENS AT LOW TEMPERATURES
When the temperature drops, oil molecules move more slowly, increasing intermolecular forces. This makes engine oil thicken and flow more sluggishly. Higher viscosity means the oil circulates more slowly through engine components, reducing lubrication. Without a stable oil film, friction rises, leading to higher wear and greater energy consumption during startup. That’s why engines often feel “sluggish” on cold winter mornings.
Beyond physical factors, the composition of the oil also affects its flowability. Mineral oils have irregular molecular structures that easily cluster at low temperatures. In contrast, synthetic oils have uniform molecular chains, allowing them to flow smoothly even below freezing. Therefore, in cold climates, synthetic or semi-synthetic oils are the preferred choice for reliable engine performance.
THE VI INDEX – A KEY FACTOR FOR OIL STABILITY
Definition and Importance
The Viscosity Index (VI) indicates how much an oil’s viscosity changes with temperature. Oils with a low VI thicken quickly in the cold and thin rapidly when hot. In contrast, a high VI means the oil maintains a stable viscosity across temperature ranges. This stability is crucial for continuous lubrication and reliable engine protection. Mineral oils typically have a VI of 90–120, while synthetic oils can reach 160 or more.
Improving VI with Additives
To enhance stability, manufacturers add Viscosity Index Improvers (VII) — special polymers that expand at high temperatures and contract at low ones. This reversible behavior keeps the oil flexible without altering its chemical structure. As a result, during cold starts, the oil film forms faster, reducing friction, wear, and startup resistance while ensuring smoother engine operation.
Impact on Engine Efficiency
A high VI not only keeps viscosity stable but also maintains a continuous lubrication film, even when temperature fluctuates. This minimizes deposits, improves fuel economy, and enhances engine durability. Oils with high VI help engines perform efficiently under both cold and hot operating conditions, extending overall service life.
HOW TO CHOOSE OIL WITH THE RIGHT VI FOR COLD CLIMATES
When selecting engine oil, consider both the SAE grade and the VI index, not just the brand. Oil labels such as 0W-40, 5W-30, or 10W-40 indicate flow performance. The letter W (Winter) represents cold-start viscosity. The smaller the number before the W, the better the oil flows at low temperatures. For example, 0W-40 can operate at –35°C, while 15W-40 performs best around –15°C.
Users should also check API or ILSAC certifications. Oils meeting API SN, SP or ILSAC GF-6 standards are formulated with modern additives that enhance VI, improve oxidation resistance, and increase thermal stability. Such oils not only start easily in cold weather but also reduce friction, keep engines cleaner, and save fuel in long-term use.
THE VI INDEX – THE EVOLUTION STORY OF MODERN LUBRICANTS
In the past, traditional mineral oils had significant drawbacks: thickening in winter and thinning too much in summer. Engine maintenance depended heavily on weather conditions, often leading to reduced durability and higher maintenance costs. The invention of synthetic oils and polymer-based VI improvers revolutionized the lubricant industry.
Today’s advanced lubricants can reach VI values above 200, allowing stable operation from –40°C up to over 100°C. Engines start faster, run smoother, and consume less fuel. Moreover, these oils resist evaporation and reduce CO₂ emissions, contributing to sustainability goals in the modern lubricant sector.
Explore Lubricant Additives Officially Distributed by Vichem
VICHEM – OFFICIAL DISTRIBUTOR OF MINGLAN LUBRICANT ADDITIVES IN VIETNAM
Vichem is the authorized distributor of Minglan lubricant additives in Vietnam, fully compliant with local legal regulations. We not only provide high-quality products but also deliver international-standard after-sales services. Our services include:
- Technical consultation: Guiding customers to select products suitable for their specific applications.
- Color and performance testing: Our laboratory team conducts quality checks to ensure results meet design requirements.
- Application support: Assisting clients in integrating additives into production lines for optimal performance.
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