Short answer
HTHS stands for high-temperature high-shear viscosity: viscosity measured at 150 °C and a shear rate of one million reciprocal seconds, conditions meant to resemble a running engine’s bearings and piston rings. It is reported in centipoise (cP), also written mPa·s. SAE J300 sets a minimum HTHS for each grade, and some specifications, such as API FA-4, set a range.
Why kinematic viscosity is not the whole picture
Kinematic viscosity is measured with oil flowing gently through a glass tube. Inside a bearing the oil film is sheared extremely fast, and the long polymer molecules used as viscosity index improvers in multigrade oils line up and temporarily thin out. Two oils with the same 100 °C viscosity can therefore behave differently in a bearing.
HTHS captures that temporary thinning at a temperature close to the hottest parts of the engine, which is why engine designers treat it as a closer stand-in for real film thickness than the 100 °C number.
How it is measured
The most common method is ASTM D4683, which uses a tapered bearing simulator: a slightly tapered rotor spinning inside a matching stator at 150 °C. ASTM D4741 (tapered plug viscometer) and ASTM D5481 (multicell capillary viscometer) are recognized alternatives.
Where HTHS limits appear
SAE J300 sets floors by grade, such as 2.6 cP for 20 grades, 2.9 cP for 30 grades and 3.5 or 3.7 cP for 40 grades. Heavy-duty diesel categories use it to separate oils: API CK-4 requires at least 3.5 cP, while API FA-4 is held between 2.9 and 3.2 cP.
European ACEA sequences and many automaker specifications also set HTHS bands, often to separate full-protection grades from fuel-economy grades. When a European manual lists a specific approval, that approval already carries the HTHS requirement.
The trade-off engine makers weigh
Lower HTHS reduces friction, which helps fuel economy and emissions, while higher HTHS keeps a thicker film under load. Each engine is designed and validated around a particular range, which is why the manufacturer’s grade and specification, rather than the highest number available, is the guide.
Frequently asked questions
Is a higher HTHS always better?
No. An engine designed for a lower-HTHS oil may see higher friction and different oil pressure behavior with a much thicker one. Use the grade and specification the manufacturer lists.
Where can I find an oil’s HTHS value?
It is rarely printed on the bottle. Product data sheets often list it alongside kinematic viscosity and viscosity index.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].