Technical guide
Engine oil viscosity explained: what 5W-30 and 15W-40 mean
Two numbers, one letter, and most of the confusion in the oil aisle. Here is exactly what SAE 5W-40 is telling you.
Written by the AG Ethiopia technical team · Last reviewed 2026-09-21
The W is for winter, not weight
SAE 5W-40 is two measurements in one label. The 5W describes how the oil behaves when cold: how easily it pumps and flows at start-up. The W stands for winter. A lower number means better cold flow — 0W flows more freely than 5W, which flows more freely than 20W.
The 40 describes viscosity at operating temperature, around 100 °C. A higher number means a thicker film when hot. That is the number protecting bearings once the engine is working.
So a multigrade oil behaves like a thin oil when cold and a thicker one when hot — which is the opposite of what an untreated oil does. That behaviour comes from viscosity index improvers, and it is why multigrades replaced monogrades almost everywhere.
The grades compared
| Grade | Cold start | Hot film | Typical use |
|---|---|---|---|
| SAE 5W-30 | Excellent | Moderate | Modern petrol cars, fuel economy, start-stop |
| SAE 5W-40 | Excellent | Strong | Turbocharged petrol, light diesel, sustained load |
| SAE 10W-40 | Good | Strong | Mixed petrol and light diesel fleets |
| SAE 15W-40 | Moderate | Strong | Heavy-duty diesel, trucks and buses |
| SAE 20W-50 | Limited | Very strong | Higher-mileage engines, hot heavy-duty service |
| SAE 40 / 50 | Poor | Very strong | Generator sets, older engines at constant load |
Thicker is not safer
The most common mistake is treating a heavier grade as extra insurance. It is not. An engine's oil pump, galleries and bearing clearances were designed around a specific viscosity. Fit an oil that is too thick and it reaches the top end more slowly at start-up — which is precisely when most wear happens — and costs measurable fuel once running.
There is one legitimate case for going heavier: a worn engine whose clearances have opened up, where a thicker film restores oil pressure the original grade can no longer maintain. That is a repair deferral, not an upgrade, and it does not apply to a healthy engine.
Why multigrades can lose their grade
The polymers that give a multigrade its spread are long molecules, and they can be mechanically chopped by gears and high shear. An oil that shears down is no longer the grade on the label — a worn 15W-40 can end up behaving like a 30. This is called shear stability, and it is one of the things that separates a well-formulated oil from a cheap one.
It is also why monogrades survive in generator sets. At constant load and constant temperature there is no need for multigrade behaviour, and without a viscosity modifier there is nothing to shear down.
This guide explains the principles. It cannot see your machine’s handbook, its service history or its operating temperature — where the two disagree, the builder’s specification governs. Ask our technical team
01 / Questions
Frequently asked.
Winter. It describes how easily the oil flows when cold at start-up. A lower W number means better cold flow, so a 5W reaches the top of the engine faster on a cold morning than a 20W does.
Neither is better in the abstract — they suit different engines. A 5W-30 starts faster and costs slightly less fuel; a 10W-40 holds a thicker film when hot. Use the grade the handbook specifies.
Only if the builder lists it as acceptable. The hot-viscosity behaviour is similar but the 15W has noticeably worse cold flow, which lengthens the dry-running window at first crank.
Too thin for the engine and the oil film can break down under load. Too thick and the oil reaches the top end slowly at start-up, costs fuel, and in cold conditions can leave parts unlubricated for several seconds.
Put it into practice.
Tell us the vehicles or machines you run and we will match each one to a grade, a pack size and a realistic service interval.