Engine oil viscosity refers to how easily oil pours at a specified temperature. Thin oils have a water-like consistency and pour more easily at low temperatures than heavier, thicker oils that have a more honey-like consistency. Thin is good for easier cold weather starting and reducing friction, while thick is better for maintaining film strength and oil pressure at high temperatures and loads.
How is viscosity rated? The Society of Automotive Engineers (SAE) developed a scale for both engine (motor oil grades) and transmission oils.
What are motor oil viscosity ratings?
You probably wonder what is the importance of viscosity in your engine.
The viscosity will determine how easily the oil is pumped to the working components, how easily it passes through the filter, and how quickly it drains back to the engine. The lower the viscosity, the easier all this will happen. That is why cold starts are so critical to an engine because the oil is cold, and so relatively thick.
A fluid’s viscosity is important because it is directly related to its load-carrying capabilities. The greater a fluid’s viscosity, the greater the loads it can withstand. The viscosity of a fluid must be adequate to separate moving parts under normal operating conditions (temperature and speed).
Knowing that a fluid’s viscosity is directly related to its ability to carry a load, one would think that the more viscous a fluid, the better it is. The fact is, the use of a high-viscosity fluid can be just as detrimental as using too light an oil.
It is the rate of change of viscosity between 2 temperatures. The lower the Viscosity Index, the more the drop in viscosity as the oil warms up. The higher the VI value, the less the drop in viscosity as the oil warms up. Generally speaking, the less it changes, across a range of temperatures, the better.
The VI scale goes from 0 to 100, with 0 being the worst, and 100 being the best. New products are now better than when the scale was first made, so some new products have scores as high as 400.
As oil heats up, its ability to provide effective lubrication diminishes. As this decreases, friction and heat increase, which can lead to mechanical failures. Therefore, the longer an oil can retain its optimum viscosity, the more effectively it will lubricate an engine and prevent damage. In this way, viscosity index can be a useful way of judging an oil's overall quality, and is an essential piece of information when selecting an oil for heavy-duty use involving wide variations in temperature.
High or Low Viscosity Index ?
A lubricant may merit having a high VI for one or more of the following reasons:
- The optimum viscosity is not known
- Varying loads and speeds exist
- Varying ambient temperatures exist
- To boost energy efficiency
- To boost oil service life (lower average temperature)
- To boost machine service life (fewer repairs and downtime)
Cheaper, lower VI lubricants may make sense if
- Speeds and loads are constant
- Temperature is constant (constant ambient temperature or a heat exchanger is in use)
- The optimum viscosity at the operating temperature is known and is consistently achieved
An oil’s VI can also tell you useful information about a lubricant’s formulation, including the type and quality of base oils. For instance, highly refined and pure mineral oils will have correspondingly higher VIs. Certain additives, such as viscosity-index improvers and pour-point depressants, influence VI as well.
What oil do I use for my car? Your engine needs oil that is thin enough for cold starts, and thick enough when the engine is hot. Since oil gets thinner when heated, and thicker when cooled, most of us use what are called multi-grade, or multi-viscosity oils.
What viscosity oil to use in modern cars?
Today, multi-viscosity oils are used almost exclusively over monograde oils since they function best in changing climates. An SAE 30 motor oil, for example, is consistent with the viscosity of, say, a 5W-30 oil when both are at 210 degrees, but the latter offers better performance in the cold as well. Multi-viscosity oils also contain additives that keep the oil from thinning out as its heated.
What kind of oil for my car if it’s a modern one?
Most modern car engines use a 5W30 or 5W40 oil. Modern oils have a lower viscosity. This means they're thinner, helping the engine turn over more easily, and ensuring that the oil can circulate quickly around the engine soon after start up. Thinner oil helps fuel consumption too.
What oil for my car if it’s an old one?
Older cars do not run well with these oils, as they are not designed to such tight tolerances. Cars that are 10 years old or older were designed to work with 10W30 or 10W40 motor oil.
What motor oil to use if it’s a high mileage car?
High-mileage cars that have over 100,000 miles may even need a 20W50 oil.
The term oil viscosity chart is not specifically linked to the lubricants business: we speak of viscosity chart to compare some liquids’ viscosity in each the 3 standard measurement units.(Centipoise cp, Centistokes cSt, Saybolt Second Universal (SSU) For example, when you order a liquid filling machine, you can use this viscosity chart to compare your liquid viscosity with one of these liquids to help your supplier understand your needs.
Relating to oil, we rather use the following 2 terms:
- Viscosity Converting Chart or viscosity conversion chart: it is a viscosity converting table between units like Centipoises, milliPascal, CentiStokes and SSU
- Viscosity Comparison Chart :it gives the viscosity in Centipoises according to the type of oil (motor oil, gear oil,…)
Which recommended lubricant for my vehicle?
Our Lube advisors help you to find the best oil for your model and brand of vehicle.
You can have a look to our Lube advisors below for the following countries:
Otherwise, if you are looking for an engine oil guide, you can search for it in our online lubricants guide and catalog on http://catalog.lubricants.total.com/automotive
You’ll be able to find the recommended motor oil, gear oil, axle oil, brake fluid… for many automotive applications: cars, scooters, motorcycles, trucks and buses, public works vehicles, leisure boats and agriculture equipment. You’ll be able to check all our oil specs and official homologations.
