Oil viscosity is one of the most critical factors in protecting an engine. It governs the oil’s ability to maintain a lubricating film between moving parts, minimising metal-to-metal contact, reducing friction, and preventing premature wear.
However, viscosity is not a fixed property. It varies with temperature, which means that an oil must be carefully selected to provide consistent protection across the full operating range of the engine.
Understanding Viscosity Behaviour
Viscosity is a measure of a fluid’s resistance to flow. In engine oils, this directly affects the ability of the lubricant to form and maintain a protective layer.
- At low temperatures, oil becomes thicker (more viscous), increasing resistance to flow. This can delay lubrication during cold starts, increasing wear.
- At high temperatures, oil becomes thinner (less viscous), which can compromise the oil film’s strength and allow metal surfaces to come into contact.
The rate at which viscosity changes with temperature is known as the viscosity index. Oils with a higher viscosity index exhibit less change across temperature ranges and offer more stable performance under real-world conditions.
Risks of Viscosity Outside Specification
When oil viscosity deviates from the recommended specification, it can lead to a cascade of mechanical and chemical issues.
- Low viscosity can result in:
- Insufficient oil film thickness, leading to metal-to-metal contact
- Accelerated wear of bearings, cams, and cylinder walls
- Increased oil consumption and volatility
- Shortened oil life due to shear and thermal degradation
- High viscosity can lead to:
- Poor cold-start performance and delayed lubrication
- Increased energy losses from fluid friction
- Elevated oil temperatures due to internal drag
- Sludge formation from oxidation and thermal breakdown
Application-Specific Selection Is Critical
Selecting an oil purely based on manufacturer recommendations may not account for the specific conditions in which a vehicle or machine operates. Factors such as load cycles, operating temperature extremes, idling frequency, and fuel type all influence oil performance in service.
This is particularly important in heavy-duty applications such as refuse collection, municipal fleets, and stop-start urban operations, where engines are frequently exposed to harsh conditions and extended idle times.
The oil must be capable of maintaining appropriate viscosity not just in ideal conditions but across the entire duty cycle.