Engine oil: Oil viscosity index

After a brief review of the use of engine oil, we will now move on to one of the most important concepts: oil viscosity index.

Oil viscosity index explained

Viscosity is simply the resistance of a fluid to flow. A higher viscosity implies a slower flow and a thicker fluid.

The viscosity changes with temperature to ensure protection and operability under all circumstances. Thus, the viscosity index measures the ability of an engine oil to resist becoming thinner at high temperatures.

To be able to change the viscosity index, conventional oils incorporate additives, usually plastic polymers. Some synthetic oils are able to achieve this variation without additives.

The SAE (Society of Automotive Engineers) established a numerical code to represent the viscosity of oil at both hot and cold temperatures.

The number before the W represents the oil viscosity at lower temperatures, while the second number (after the W) indicates the viscosity at operating temperatures.

For example, this means that 5W30 engine oil has less viscosity at lower and higher temperatures when compared to 10W40.

You should be aware of climate changes in order to keep your engine fully functional. Here you can see the recommended oil for each ambient temperature:

SAE specifications for oil viscosity index

Ambient temperature in ºC

If you are interested, you can see more information regarding SAE specifications here.

But be careful: transmission oils have a different SAE numbers. For example, a 75W90 transmission oil doesn't have a higher viscosity than a 15W40.

Oil specifications - ACEA vs API

Apart from the viscosity index, there are other factors to consider when choosing an engine oil for your vehicle.

ACEA specifications

The ACEA (Association des Constructeurs Européens d'Automobiles) has its standard regarding the specifications of  oils. They put oils through a series of tests, which get updated when new legislation, new technologies or other reasons demand change.

The ACEA specifications cover passenger and commercial engines, divided into the following sections:

  • A/B: Petrol and Diesel engines. They are usually combined in the specification. For example A3/B3 or A3/B4.
  • C: Catalyst compatible oils, which also extend the DPF life.
  • E: Heavy duty diesel engines

You can find more details about the differences between all specifications in the official ACEA PDF, specifically on page 4 and 5.

ACEA vs API oil specifications

API specifications

On the other hand, we have API (American Petroleum Institute) specifications. They use a two letter specification, dividing them into:

  • S for Petrol engines
  • C for Diesel engines
  • F for modern Diesel engines (introduced in 2017): this new specification aims at reducing fuel consumption

The second letter of the specification indicates its quality. The lowest quality is the letter A, and each letter after A represents better quality. For example, an API SN oil is better than an API SJ. As with ACEA specs, the specifications can be combined when the oil is suitable for both diesel and petrol engines. For example, an engine oil can be SM/CF.

You can learn more about API specifications on the API official site.

Brand specifications (OEM, Original Equipment Manufacturer)

It can also happen that manufacturers create their own specific set of requirements for their engines. These oils meet SAE specifications but the manufacturers add some extra conditions. Consequently, the oil needs to pass extra tests like the Volkswagen VW 507 or the BMW Longlife 01.

Stay tuned!

In the coming weeks we will talk about synthetic vs conventional oils and how additives can improve a motor oil's performance.