The Valvoline blog frequently addresses oil viscosity as a critical factor in engine oil selection.
Whether your application involves passenger cars, motorcycles, boats, or heavy-duty fleet vehicles, selecting the correct oil viscosity is fundamental to both engine longevity and performance.
If terms like 'oil viscosity,' 'viscosity grades,' or 'viscosity index' still seem unclear, there is no need for concern. We have put together this concise guide to help demystify the viscosity of oil.
Engine oil becomes thinner as temperatures rise and thicker when temperatures drop. This behavior is precisely why viscosity ranks among the most critical oil properties to evaluate.
In simple terms, viscosity describes how strongly an engine oil resists flowing.
To illustrate: a thin engine oil carries lower viscosity, while a thick one carries higher viscosity. Oils with lower viscosity pour more readily in sub-zero conditions than their high-viscosity, thicker counterparts.
Low-viscosity oils reduce friction between engine components and support quicker cold-weather starts. High-viscosity oils, by contrast, preserve film thickness and strength, making them well-suited for industrial and heavy-duty machinery that operates under elevated temperatures and pressures.
The viscosity of engine oil influences the performance of vehicles, machines, and equipment across several dimensions.
First, viscosity governs film thickness. A primary function of engine oil is forming a robust film that keeps two metal surfaces apart, preventing direct metal-to-metal contact. Under high-pressure conditions, oil viscosity increases, sustaining full-film separation even in the most demanding operating environments.
Viscosity also determines film strength, which enables the oil to reduce friction and limit wear. When temperatures and loads climb to extremes, film strength rises accordingly.
By enhancing both film thickness and strength, oil viscosity lowers a machine's or vehicle's susceptibility to particle contamination and slows its rate of wear.
Beyond these effects, motor oil viscosity also influences several additional performance characteristics:
Two primary categories of oil viscosity warrant attention — dynamic and kinematic.
Dynamic oil viscosity, also referred to as absolute viscosity, describes how an engine oil behaves when subjected to pressure, making it a measure of force. This property is expressed in centipoises (cP).
Kinematic viscosity describes how oil flows when gravity acts upon it, which is why it is defined as the rate of oil flow. It is measured in centistokes (cSt).
Different oil types do not respond identically to temperature shifts. As noted above, the composition and quality of an oil determine how significantly its viscosity drops at elevated temperatures. This temperature-driven change in viscosity is known as the viscosity index (VI).
Calculating an engine oil's viscosity index reveals whether the lubricant is suited to the temperature range at which the engine operates.
To determine the viscosity index, oil viscosity is measured at two reference points: 40 degrees C and 100 degrees C.
The base oil within a lubricating formulation has a direct bearing on its viscosity index. Standard mineral oil, for instance, achieves a VI of up to 100. Refined mineral oils reach a viscosity index of 120, whereas synthetic oils can attain a VI as high as 250.
A higher viscosity index signals that the lubricant experiences less change in viscosity as temperatures fluctuate.
The Society of Automotive Engineers developed a standardized rating scale for both motor and transmission oils.
The leading number on the label reflects the oil's viscosity at -17.8 degrees Celsius, followed by the letter W, which denotes winter.
The trailing number describes the oil's viscosity at 100 degrees Celsius and reflects its ability to resist thinning under heat.
Consequently, when choosing engine oil for your vehicle, it is important to account for local ambient temperatures and your geographic region.
On cold winter days, thinner oils are preferable since they are less prone to thickening at low temperatures. Their improved flowability in freezing conditions can assist with faster engine starts. Examples of such oils include 0W-20 and 5W-30 grades.
In tropical or hot climates, thicker oils that retain their body at high temperatures are the better choice. Products such as 15W-40 and 20W-50 are formulated specifically for these conditions.
Viscosity stands as one of the most important engine oil characteristics to evaluate. To identify the correct lubricating solution for your engine, always follow the viscosity grade specified by your original equipment manufacturer. Matching oil to your engine type and operating environment will extend the service life of your vehicles, machines, or equipment while lowering the likelihood of expensive repairs.
Still uncertain about which oil viscosity is right for your engine? Reach out to our team of experts — we are here to help.