As the drive for increased fuel efficiency and reduced greenhouse gas emissions continues, understanding the differences between engine lubricant viscosities is becoming more essential.

When people talk about engine lubricant, the tendency might be to talk in terms of something like a “5W-30” or “10W-30” lubricant due to this being dominant on the bottle or can. This is classed as the viscosity grade and doesn’t tell the full story.

Engine lubricants are classified using the Society of Automotive Engineers (SAE) J300 oil viscosity classification system. Multi-grade engine lubricants, such as SAE 5W-30 or 10W-30, must meet both low temperature and high temperature viscosity requirements. In a SAE 5W-30 engine lubricant:

  • “5W” refers to low-temperature viscosity (the “W” denotes winter) and is an indication of how easily the lubricant will flow in cold start-up conditions. At a given temperature a 5W lubricant has a lower viscosity than a 10W lubricant so flows faster during critical start-up. This is sometimes referred to as cold cranking simulator (CCS) viscosity.
  • “30” refers to the high temperature viscosity at 100°C, therefore replicating an engines operating temperature.

This is known as an oils kinematic viscosity (KV)—the time it takes for a fixed volume of lubricant to flow through a tube under the force of gravity.

The above solely talks about viscosity grade, however there is another important type of viscosity associated with an engine lubricant: high temperature high shear (HTHS) viscosity.

In an operating engine, the lubricant is exposed to more than just gravity, the lubricant is required to lubricate and protect engine components under hotter and more severe operating conditions. HTHS dynamic viscosity is the current industry standard that best predicts fluid behavior in these operating conditions.

HTHS viscosity is not new, with SAE J300 limits defined as far back as the early 1990s. HTHS measures the lubricant’s resistance to flow at elevated temperatures under constant shear and high speeds between moving parts which have narrow tolerances—the lower the measured number, the lower the resistance to flow and the higher the anticipated fuel efficiency benefits.

Original equipment manufacturers (OEM) are moving to lower HTHS viscosity engine lubricants for their latest engine hardware designs in order to deliver greater fuel efficiency.

It’s important to understand that engine lubricants can have the same viscosity grade but different HTHS viscosity. As such, it is vital to use the correct OEM recommended lubricant to provide the necessary engine protection as well as fuel efficiency benefits.

The next article in this series will discuss why HTHS viscosity matters.