SAE¹, ACEA² and car manufacturers in their internal standards specify the minimum viscosity value of engine oils at an oil temperature of 150°C and a shear rate of 106 s-1. This is HTHS viscosity (high temperature high shear).
By specifying limit values for HTHS, they ensure that multipurpose engine oils with VI improvers provide the required lubrication even at high oil temperatures and high shear rate (high rpm).
The limit value for HTHS was defined again by ACEA in January 1996 and it is included in current standards:
A1/A5 / B1/B5
min. 2.9 up to max. 3.5 mPa·s (low HTHS value)
A2/A3 / B2/B3 / E2/E3/E4/E5
3.5 mPa·s (high HTHS value)
The requirements of car manufacturers, e.g. Volkswagen, Mercedes-Benz, Opel, Ford or BMW, specify high or low HTHS values depending on the type of engine.
HTHS is measured in a rotation viscometer using the CEC-L-36-A-97 method.
¹ SAE = Society of Automotive Engineers
² ACEA = Association des Constructeurs Europeens de l’Automobile = Association of European Car Manufacturers