Oil viscosity explained

Oil viscosity explained

Viscosity refers to how thick your engine oil is as it begins to flow through the engine at start-up. It is the single most important performance property of a lubricant.

“If the lubricant is too thick, it flows more slowly (like molasses), creating more friction and thus negatively impacting the efficiency of equipment,” explains Shell's Product Application Specialist, Dr Robert Profilet.

He adds: “If it’s too thin (like water) and moves too freely or quickly, it does not form a sufficient film to separate moving parts, wearing out machines more quickly.”

An oil’s viscosity will change with shifts in temperature. As it gets hot, its viscosity drops. As it cools, its viscosity increases.

Every engine oil has an SAE (Society of Automotive Engineers) viscosity. This number is clearly written on product or technical data sheets, as well as the product's packaging – because it’s a legal requirement.

Shell Rotella ranges

How do you ‘read’ viscosity?

Each motor oil label will vary in design, but most will contain one or two numbers specifying the oil’s viscosity grade. On a bottle of multigrade engine oil, like Shell Rotella® T6 Multi-Vehicle, for example, you’ll see two numbers: 5W-30.

The first number (5W) refers to how the oil flows when the engine is cold. The second number (30) indicates how the oil flows when the engine is hot. The “W” stands for “winter.”

So, for example, a 5W-40 engine oil and a 15W-40 engine oil will have the same high-temperature flow characteristics (40), but the 5W- will flow better and faster at lower temperatures than the 15W-. Choosing an oil which operates at the right temperature range can be beneficial, depending on your engine operating conditions.

Monograde or straight-grade oil, on the other hand, contains only one viscosity. For example, SAE 30. Straight-grade oils are typically recommended for older vehicles manufactured before multi-viscosity oils were available, prior to 1970.

Shell Rotella ranges

Why lower viscosity oil can be better for your engine

The smaller the first number, the better the oil will flow when you switch on your engine. Thin, low viscosity oils flow easier to protect engine parts at cold temperatures. The faster oil flows to critical parts of the engine, the more protected they are from wear.

So a 5W-30 will flow easier than a 10W-30 at start-up temperatures. And a 10W-30 will flow easier than a 15W-40 at normal engine operating temperatures.

What’s more, lower viscosity oils can reduce your fuel costs and CO2 emissions.

No matter what you put your engine through, you need to know your oil will protect your engine under all conditions. And the right quality oil matters.


Of course, ‘lower’ is a relative term. 10W-30 is lower than 15W-40, and 5W-30 is lower than 10W-30.

Before thinking of trying one of our oils, first be sure what your OEM recommends. Not all engines work with low-viscosity oils.

Shell Rotella T5 10W-30 Synthetic Blend

Synthetic base oils and advanced additives to deliver excellent protection, even at low temperatures where the enhanced cold-flow properties help protect your engine.

Shell Rotella T5 Ultra 10W-30 Synthetic Blend

Triple Protection Plus technology against wear, deposits and oil breakdown.

Shell Rotella T6 5W-40 Full Synthetic

Shell Rotella® T6 Full Synthetic Diesel Oil provides excellent protection against harmful engine wear and sludge.

Shell Rotella T6 Multi-Vehicle 5W-30 Full Synthetic

Shell Rotella® T6 Multi-Vehicle 5W-30 performs extremely well in both diesel and gasoline engines, allowing mixed fleet customers to consolidate lubricants.

Shell Rotella T4 10W-30

Shell Rotella® T4 Triple Protection has a proven legacy of performance and is suitable for a wide range of heavy duty engine applications.

Engine icon

How do I know which oil is right for my vehicle?

Some vehicle manufacturers provide a range of recommended engine oil viscosity grades – based on the outside temperature in which the vehicle is operated, as well as the operating conditions.

Other original equipment manufacturers (OEMs) may recommend the use of only one motor oil viscosity grade. Always follow the OEM’s recommendations found in your vehicle’s owner’s manual.

Shell LubeMatch can provide recommendations based on your specific equipment.

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