What does 5w20 or 5w30 mean?

Oil Viscosity Explained

5W - Low temperature dynamic viscosity at 40F with representing the viscosity index, and W representing "winter"

30 - High temperature kinematic viscosity index at 100F

Engine oil is complex and generally misunderstood by many. While you have several Oil brands of varying quality, the main factor to consider will always be the oil viscosity. Different oils with different viscosities have varying effects on your car. Grabbing the first type of oil you see on the shelf is better than no oil at all, but selecting the correct oil will be most beneficial over the long term.

Let’s try to understand oil viscosity, how it affects your car, and what type of oil you should buy for your car.


Viscosity Measurement

Viscosity is defined as a fluid’s resistance to flow and shear. When we explain this concept to children, we say viscosity is how easy or how difficult it is for a fluid to flow. This is true in essence but engine oil viscosity is more complex than how easily it flows.


There are two types of viscosity; dynamic viscosity and kinematic viscosity.


Kinematic viscosity is what we describe to children. A more viscous liquid will flow slower while something with lower viscosity will just flow easily. A great comparison to represent kinematic viscosity is syrup and water. Water flows easily along surfaces due to its less viscous nature while syrup has more difficulty.


The high-temperature viscosity grade of an oil is based on the kinematic viscosity.


Dynamic viscosity, on the other hand, is based on how “difficult” it is for one object to move through a fluid. A Cold Crank Simulator Test is used to determine the dynamic viscosity of a fluid.


Note: The low temperature viscosity grade of an oil is based on the dynamic viscosity.


One last thing to remember is the VI or viscosity index of different oils. The VI represents the change in the performance of the oil while experiencing changes in temperature. What you need is an oil that performs consistently through the range of temperatures your vehicle experiences. The oil’s VI gets measured at 40°C and 100°C. to determine the weight designation (ie. 5w30).

How Does Viscosity Affect Your Car?

Before we discuss how viscosity can affect engine performance, we have to talk about why engines require oil in the first place.


Your car's engine is a collection of moving metal parts. These metal parts, although sturdy, can be worn down by friction if they're allowed to rub against each other without a film of lubricant. A typical engine idles at 800-1000 revolutions per minute, and generates a tremendous amount of heat. Oil acts as the buffer between the moving parts enabling smooth, friction-free operation. Oil is also great at absorbing heat and moderating the engines internal temperature.


Low viscosity oil can be considered too thin for some engines, and may not be the best performer as a lubricating film in the "buffer zone". This can affect not only the performance but also the health of your engine. 


At this point, you might think that high viscosity oil is the way to go, right? Well, not necessarily. When your oil is too viscous, it cause your engine to work harder just to move around. Just imagine swimming in syrup vs swimming in water. This can result in poor fuel efficiency and engine wear. High viscosity oils can be considered too thick to properly penetrate the tiny spaces or tolerances separating your engine's metal parts. An oil too thick can also cause friction by leaving some parts without adequate lubrication.

Selecting the oil viscosity recommended by your vehicle's manufacture is very important to avoid these issues.

So What Is The Right Oil Viscosity For My Car?

The magic handbook to determine the right oil for your vehicle should be in your glovebox!

  1. Check your car’s manual. The manufacturer already has a suggestion for the best oil type/viscosity for your car based on the climate you live in
  2. If your car does not have a manual, open the hood and look for the OIL cap. The cap should list the recommended oil viscosity (5w20, 5w30, 0w20, etc.).
  3. The weather can affect your oil performance so you might want to change oil when winter comes or when spring begins.
  4. You should understand how oils are marked. For example, if an oil is marked 5W-50, the “5W’ refers to its performance in cold weather (lower is better for winter) while the “50” refers to the performance under normal conditions (the higher value means it stays thick at higher temperatures). So if you are looking for oil to use for the winter, picking a 0W-30 over a 5W-30 oil makes sense. For the summer, you might want to pick a 5W-50 over a 5W-30 oil, but ensure you always choose an oil that is within your manufacturer recommended specifications.


Understanding oil viscosity might seem like an “advanced car owner” kind of thing but it’s really important if you want to give your car the best. The type of oil you put into your car can dictate how fuel efficient your car is, the level of performance it can provide, and its health.


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