The behavior of engine oils essentially depends on the temperature and it is important to know the influence of heat and cold on the behaviour of engine oils so that specific precautions can be taken to protect the engine. At any rate, lubricants have to provide sufficient flow, film strength, and chemical stability, whether at a cold start – sometimes as low as at -30 C – up to top operating temperatures, that are often above 100-150 C in the engine.
Most individuals accept a myth that once the oil is flowing within an engine when hot it acts more or less the same no matter the weather conditions. This is a misconception. The temperature of oil within the engine is determined by the ambient temperature as well as the engine load and it varies in real time when running. These changes vary viscosity, film-forming ability and degradation speed dynamically.
Even with a good engine oil, when not formulated well, it might not be effective in another temperature that is not in line with the particular oil. It is exactly the reason why the multigrade oils and the high-viscosity-index base stocks in the modern world were invented, so that the magnitude of these changes in properties over operating range diminishes.
The range of ambient and operating temperatures differ so substantially across geography and application that climate should muster the choice of oil. In order to select the appropriate engine oil in your market and climatic condition, choose the right engine oil for your market and climate, the starting point is always a clear understanding of how temperature influences the fluid’s physical and chemical behavior.
Why Temperature Has Such a Strong Impact on Engine Oil
The only external variable that influences engine oil the most is temperature since it dictates the level of kinetic energy of the hydrocarbon molecules that compose the base fluid.
The higher the temperature the more vigorous is the motion of its molecules. Intermolecular attractive forces (van der Waals forces) become weaker compared to this augmented kinetic energy and thus the molecules move about each other with less difficulty → viscosity decreases. In order to decrease temperature, the movement is reduced, the forces of attraction predominate, and the oil becomes significantly more obstinate to movement – viscosity increases exponentially.
This non-linear effect on temperature and flow resistance (viscosity) is essentially the cause of why a single-grade oil can practically never be used all-year-round in most climates. A test that was not used with viscosity modifiers and a carefully chosen base oil would mean that an oil will be pumpable at operating temperature but almost solid in winter on a startup.
How Engine Oil Behaves at Low Temperatures
At low temperatures, however, the performance of the engine depends on whether it receives timely lubrication at the stage of operation where it experiences the greatest wear: cold starting.
Cold start The oil temperature at cold start is the same or close to ambient temperature and its viscosity is much greater than at normal operating temperature. This higher viscosity reduces the speed at which the oil pump can build sufficient pressure to elevate the lubricant to the parts which require it to operate, to the camshaft bearings, main bearings, piston rings, and to the valve train. The delay has a direct effect of enhancing metal to metal contact and abrasive wear.
The minimum temperature at which the oil can be pumped out of the sump and pass through pick up screens and passages without cavitation or starvation is referred to as the pumpability limits. Under this threshold the oil can act more of a gel than a liquid.
When the startup becomes even more difficult to start, the issue gets worse. Lubrication The slow rate of oil flow implies that hydrodynamic lubrication cannot be rapidly established, and instead, it falls under the classes of either boundary or mixed lubrication where additive packages must counter the inability to achieve complete separation of the oil film.
| Temperature Condition | Oil Behavior | Potential Risk |
| Below 0°C | Thickened oil | Delayed lubrication |
| Extreme cold | Poor flow | Startup wear |
A deeper explanation of these dynamics appears in our discussion of temperature-viscosity behavior.
How Engine Oil Behaves at High Temperatures
Engine oil at high temperatures would be much thinned. Though better flow may help aid cooling and distribution, excessive reduction in viscosity poses a threat to the load carrying ability of the oil.
The major concern is film strength reduction. Finer oil creates a weaker hydrodynamic wedge between surfaces extended heavily (crankshaft journals, cam lobes, piston skirts). It can proceed to mixed or boundary lubrication under high shear rates and pressures and this increases the risk of adhesive wear or scuffing.
The rate of the reaction is Arrhenius scaled: The rate of the reaction is approximately doubled as temperature increases by 10 o C. Hot oil consequently drains antioxidant additives more rapidly, develops organic acid and varnish and sludge layers, and also loses neutralizing capability of combustion by-products.
The specifications of the formulations designed to suit these situations may be located in our article regarding the best engine oil in high temperature climates. best engine oil for high-temperature climates.
Temperature, Viscosity, and Lubrication Film Stability
What determines the effectiveness of any lubricant is that it should be able to form and maintain a separating film in the presence of the load existing, speed, and temperature.
The process of oil film formation depends on hydrodynamic pressure caused by the relative movements of the surface and viscosity of oil at that time. When at low temperatures increase viscosity will assist in quick film formation when flow starts but too thick prevents any flow. Film collapse and shear in a high temperature environment can be prevented by ensuring that the oil has sufficient viscosity to withstand shear and load, otherwise wear and asperity contact increases faster.
Stability of films at a variety of temperature thus holds much more significance than absolute thickness of a point. A low viscosity-temperature oil can be an excellent choice at 100 C but be a disastrous choice in the case of a cold start- or the reverse.
The real-life application of heat and cold effects on engine oil viscosity are explored further in our guide to grade selection for different climates.
Comparing Oil Viscosity Changes Across Temperature Ranges
Multigrade oils are categorized by their low-temperature cranking and pumping viscosity (the W number) and kinematic viscosity at 100 o C (the second number). These two facts can be used to demonstrate the sensitivity of common grades to temperature.
| Oil Grade | Cold Temperature Behavior | High Temperature Behavior |
| 5W-30 | Good cold flow | Moderate film strength |
| 10W-40 | Slower cold flow | Stronger high-temp film |
Above the table indicates that a 5W-30 will have a greater flow rate in cold climate conditions where the engine needs to be started, and thus the engine wears less when it is cooler, where a 10W-40 will exhibit higher levels of viscosity in hot conditions where the engine is in high demand or the higher ambient temperature.
These and other grades which change viscosity change with temperature, compare these and similar grades in detail.
Temperature Effects on Oxidation and Oil Degradation
Oxidative deterioration of engine oil is mostly caused by elevated temperatures. The overall kinetics of oxidation are exponential: the rate of cumulative depletion and polymerization of the base-oil is approximately doubled with each increase in temperature by 10 C making useful oil life itself directly proportional to the temperature.
In extreme instances (High load operation in hot climate) this will increase acidity and cause deposits, and in severe cases, polymerization causes viscosity which results in loss of anti-wear and detergency performance.
Sufficiently developed oils contain thermal stress effects by a blend of synthetic base stocks or very highly refined base stocks, strong antioxidant systems and thermal stabilizers. These are measures that increase the temperature range in which the oil can be maintained.
The variation in the degradation mechanisms has been discussed in more how engine oil behaves differently in hot and cold conditions.
Why Temperature Behavior Matters When Choosing Engine Oil
The aspect of temperature behavior does not seem a thing secondary but one of the main factors that will point to whether an oil will perform a typically reliable protection within a particular application.
In all the keenly cold places, it is better to have great low-temperature pumpability and cranking performance in order to minimize the startup wear and fuel economy. In constantly hot climates or high-duty cycle operations, oils of high high temperature viscosity retention and oxidation resistance are mandatory to regulate deposits and keep film integrity intact.
When these realities are ignored, over wear, reduced drain times, higher maintenance costs or even disastrous components may occur. Market-oriented and climate-compatible suggestions thus comprises a step towards prudent lubricant choice.
Common Misunderstandings About Temperature and Engine Oil
There still are misunderstandings related to temperature effects even in the case of advanced users.
One common one is the notion that thicker oil keeps one safer in heat. As a matter of fact, too thick an oil at operating temperature may not only raise internal fluid friction, but it may also raise operating temperatures yet again, and may also have lower pump efficiency- in some cases less protection than a well balanced grade.
The other is the reason behind the belief that cold weather only affects starting. The low-temperature flow is still bad until low-temperature to equilibrium temperature is reached by the entire lubrication system, and this might need several minutes.
Lastly, the fact that the climate issues with using OEM grade verbatim as a guideline ignores the reality that recommendations provided by the original equipment manufacturer are usually a compromise to fit in the average conditions. Even in severe climates simply changing towards a more suitable grade of viscosity within the accepted range of API or ACEA lines often results in superior long term engine life.
Conclusion — Temperature Defines Real-World Engine Oil Performance
In the actual performance of engine oil, temperature persists as the explanatory variable. None of other single factors affect flow characters, film-forming capacity, wear protection which as well as chemical stability by the same extent.
The best oils are those that are designed to have minimal variation in the important property as the entire range of the anticipated operating temperatures. To reach such stability, it is necessary to pay close attention to the quality of base oil, viscosity index improvers, and additive systems resistant to thermal and oxidative degradation.
The engineering decisions herein lie right at the heart of the reason behind the emergence of viscosity index, pour point, and high-temperature/high-shear viscosity as key specification entities- and the need to continue to learn the physics behind the influence of temperature effects on anyone tasked with the role of deciding upon lubricant selections or engine durability.