5W-30 vs 5W-40 Engine Oil: What the Numbers Mean and How to Choose

Both are 5W oils that flow the same when cold. The difference is hot: 5W-40 is thicker at operating temperature. Here is what the numbers mean and how to choose the right grade for your engine and climate.

If your engine handbook allows both, the difference between 5W-30 and 5W-40 engine oil is simple: they flow the same way when the engine is cold, and 5W-40 is thicker once the engine is hot. Neither one is "better". The right one is the grade your engine maker specifies, and this guide shows how to read that choice with confidence.

The short answer: 5W-30 vs 5W-40 engine oil

• Cold start: both are "5W" oils, so both flow easily when the engine is cold.
• Hot running: 5W-40 stays thicker at normal operating temperature than 5W-30.
• The deciding rule: use the viscosity grade written in your owner's handbook. A thicker oil is not automatically more protective, and a thinner oil is not automatically more efficient.

What the numbers in 5W-30 and 5W-40 mean

Engine oil grades follow the SAE viscosity system (SAE J300). Viscosity is how easily an oil flows. A multigrade oil such as 5W-30 carries two numbers because it has to work in two very different conditions.

The first number and the "W": cold-start flow

The number before the W (for winter) describes how well the oil flows when cold. The lower the number, the easier it flows at low temperature. A 0W oil flows more easily in the cold than a 5W oil, and a 5W oil flows more easily than a 10W oil. Since 5W-30 and 5W-40 share the same "5W", their cold-start behaviour is in the same class.

The second number: thickness at operating temperature

The second number is the viscosity grade at engine operating temperature, measured at 100 °C. Under the SAE system:

• Grade 30 covers a kinematic viscosity at 100 °C from 9.3 up to (but not including) 12.5 mm²/s.
• Grade 40 covers a kinematic viscosity at 100 °C from 12.5 up to (but not including) 16.3 mm²/s.

So a 40-grade oil is thicker than a 30-grade oil when hot. That one step is the whole difference between 5W-30 and 5W-40.

5W-30 vs 5W-40 at a glance

When 5W-30 is the right engine oil

Choose 5W-30 when the handbook, the oil filler cap or the maker's service schedule names it. Many modern petrol engines are built with fine tolerances and are specified for 30-grade or thinner oils. A thinner oil moves through narrow passages quickly and creates less internal drag. Fitting a thicker oil than specified in such an engine can work against the design.

When 5W-40 is the right engine oil

Choose 5W-40 when the handbook lists a 40-grade oil. Some engines, including many turbocharged petrol engines and many diesel engines, are specified for 40-grade oils because the maker expects higher operating temperatures or heavier loads. If the handbook lists both 30 and 40 grades as approved, the 40 grade is a reasonable choice for sustained high temperatures and heavy loads, and the 30 grade for mainly light, short or urban driving.

Which grade for a hot climate?

In hot countries the oil spends most of its life hot, and cold starts below freezing are rare. For that reason the second number matters more than the "W" number. Still, the same rule applies: heat does not override the handbook. If your maker approves both grades, 5W-40 gives a thicker film at high temperature, which suits hot weather, heavy loads, towing and stop-and-go traffic. If the maker specifies only 5W-30, stay with 5W-30 and shorten the oil change interval in severe conditions rather than changing the grade.

Synthetic, semi-synthetic or mineral: a separate choice

Viscosity grade and base-oil type are two different decisions. Fully synthetic, semi-synthetic and mineral oils are all sold in 5W-30 and 5W-40 style grades. The viscosity grade tells you how thick the oil is. The base-oil type tells you how it is made and how it resists heat, oxidation and ageing. Many 5W oils use synthetic base oils because these flow well in the cold and resist heat and ageing.

Viscosity is not quality: read the performance rating too

A grade such as 5W-30 says nothing about performance. Look for the performance standard next to it:

• API categories for petrol engines, such as API SN and API SP, and for diesel engines, such as API CI-4 and API CK-4.
• ACEA sequences, common on European vehicles.
• Maker approvals listed in your handbook.

Always match both the viscosity grade and the performance standard. A correct grade with the wrong rating is still the wrong oil.

How to choose between 5W-30 and 5W-40 in four steps

1. Read the handbook. Find the viscosity grade and the performance standard it requires.
2. Check the approved list. If it lists only one grade, that is your answer. If it lists several, pick by climate and load.
3. Match the rating. Confirm the API or ACEA standard on the pack matches what the handbook asks for.
4. Buy from a source you can verify. Check that the pack has a clear label, a batch code and a data sheet available on request.

Common mistakes to avoid

• Going thicker "to protect an old engine" without checking whether the maker allows it.
• Reading the first number as thickness when hot. The 5 in 5W is about cold flow only.
• Mixing grades casually. Topping up with a similar oil in an emergency is acceptable, but do a full change with the right grade as soon as you can.
• Ignoring the performance rating and choosing only by the viscosity numbers.
• Stretching change intervals in dusty, hot or heavy-load conditions.

TexLube engine oils

TexLube makes engine oils in the United Arab Emirates for petrol and diesel engines. The Garnet family covers petrol engine oils from fully synthetic to mineral; browse the fully synthetic engine oils or see an example such as Garnet SR1 SAE 5W-30 API SP. For diesel engines, see the diesel engine oils. Every product page carries a data sheet and a safety sheet, so you can compare the grade and the performance standard with your handbook before you buy. If you are not sure which grade your vehicle needs, contact the TexLube team with the make, model and year.