What HTHS viscosity measures
Most viscosity figures, including the SAE grades on the bottle, are measured under gentle conditions. HTHS (high-temperature high-shear) viscosity is different: it measures how thick the oil remains at 150°C while it is being sheared very rapidly. These conditions are designed to mimic the harshest places inside a running engine, such as heavily loaded bearings and the gap between the piston ring and cylinder wall.
The result is reported in centipoise (cP) and tells engineers how well the oil maintains a protective film exactly where metal-to-metal contact is most likely.
Why it matters and the trade-off
A higher HTHS value means the oil stays thicker under heat and shear, which generally supports a more durable oil film under heavy load. A lower HTHS value lets parts move with less internal resistance, which reduces friction and can improve fuel economy.
This is a genuine trade-off, not a case where one direction is simply better. Modern engines are frequently designed around lower-HTHS oils to meet efficiency targets, with bearings and clearances matched to that thinner film. Using a much higher HTHS oil than intended does not automatically add protection and may work against the engine’s design.
How HTHS ties to specifications
You normally do not select HTHS as a number. Instead it is built into the specifications a finished oil meets. ACEA sequences and many OEM approvals set minimum (and sometimes maximum) HTHS limits, so an oil carrying the right specification already has an appropriate HTHS for that engine family.
This is why HTHS belongs to the specification side of oil choice, not the grade side. Match the grade and the specification your owner’s manual lists, and the correct HTHS comes with it.