What do the numbers on engine oil mean?

Oil Viscosity

They relate to oil viscosity or thickness of the oil at a given temperature. Oil flows slower when it is colder as it is thicker like honey. It flows faster when it gets hot and thins out, much like water. This viscosity is also known as the engine oil weight. The heavier the weight, the thicker it is, the lighter the weight the thinner the engine oil.

The reason for these different viscosities is that the oil needs to protect the engine on start up when it is below zero on a freezing cold morning and still protect it on a 37-degree day, while pulling a loaded trailer through the hills. They are both temperature extremes and the oil should ideally have a constant viscosity in both cases.

Engine designers take the oil viscosity into account when developing the engine. The oil lubricates the moving metal parts and without it the engine would fail rather quickly. Between these parts are gaps or tolerances. If the oil is too thin it will run out too quickly, if too thick, it will struggle to fill the gap. Both extremes will cause premature wear on the engine.

Engine seals are also designed to work with this viscosity of oil. That is why it is important to follow the manufacturer’s recommendations on oil viscosity for your engine. There are slight variations. For example, if you live in the hot desert climate of Egypt, the recommended viscosity of oil may differ compared to if you were living in the cold wastes of Alaska.

The Engine Oil Numbers

The Society of Automotive Engineers (SAE) came up with a method to grade oil viscosity so automobile engineers globally would have a set standard that could be used when choosing oils. Originally there was just a single number — i.e., SAE 30, which is still used today in a lot of smaller engines like lawn mowers. However, they soon realised that due to the vast temperature differences in many places, an oil had to perform in cold and hot conditions.

This is where the current multi-grade oil numbers come from. For example, a 15W–40. 15W is one grade and 40 is the other. A 15W-40 oil is the same no matter what company made it or which country it came from. They tested how well oil flowed at certain temperatures and created a set of oil grades for cold climates and another for hot climates.

This is not a temperature but a grade. The 2 gradation tables are shown below. Climate does impact how hot or cold an engine can become before it runs or during its run time which impacts the oil. 

This table deals with cold conditions and how well oil flows in temperatures of extreme cold. This is the first part of the number on the oil bottle and ends in a W for ‘Winter’. You then know it is the cold grading. The cold grades are highlighted in blue, 0W is the thinnest oil, 25W is the thickest or heaviest weight.

This table deals with hot conditions and how well oil flows in temperatures of extreme heat. This is the second part of the number on the oil bottle has no letter at the end, which lets you know it is the hot grading.  The hot grades are highlighted in red, 20 is the thinnest oil, 60 is the thickest or heaviest weight.

Engine Oil Numbers Explained

Now that we have covered the background of the multi-grade oils we have today, let’s break it down a bit, as it can get confusing.

In terms of the cold table, cranking viscosity lets us know the viscosity of the oil in the crank shaft bearings when we start the engine at a low temperature, while the pumping viscosity tells us the viscosity of the oil when the oil pump moves it through the engine.

For the hot table, kinematic viscosity is a measure of the oil’s flow under gravitational forces, while shear rate relates to the speed of the liquid’s movement.

Don’t be too concerned about this data though, that was to show there is science behind the viscosity values and it is not just made up by the oil manufacturers. The important thing to take away is the meaning of the grade numbers.

Above, we have highlighted another 2 oils. The 20W-50 on the right has been around for a long time and was the only choice for a lot of years, while the 5W-30 is a lot more recent.

Overall 5W-30 is a much thinner oil than 20W-50 in cold and hot conditions and it is part of a growing trend to make oils thinner. Why? There is technically nothing wrong with a 20W-50, it worked fine for years and still does. 

The answer is fuel economy. The thicker the oil’s viscosity, the more friction it creates between the moving engine parts it lubricates, and the thinner it is, the less friction it has. When you think about all the places the oil goes to lubricate engine components, it all adds up.

Friction creates a resistance to the rotating parts. Having a thinner oil decreases this resistance so the engine spins or rotates more freely. This means it needs less power to accelerate and keep it rotating, which in turn gives better fuel economy for your engine.

Head Into Repco for Your Oil Needs

With petrol prices constantly going up and fuel economy directly tied to your engine oil choice, picking the correct oil for your vehicle is more important than ever. If you need engine oil or other automotive advice, head into your local Repco store and talk to one of our staff and get the right oil for you.