HTHS is dynamic viscosity measured at high temperature and high shear, normally 150 °C for the engine-oil classifications discussed here. It is not a score for engine protection. SAE J300 includes HTHS requirements; ACEA and OEM specifications may add constraints.
What the source establishes
ACEA 2023 light-duty sequences, page 7, specify HTHS limits at 150 °C: C3 requires at least 3.5 mPa·s, while C2 requires at least 2.9 mPa·s. Those thresholds belong to different specification packages and do not make C3 a universal upgrade over C2.
Match both the SAE grade and the required specification. If a product data sheet gives HTHS, check the test method and units before comparing values. Equal SAE grades can contain different measured HTHS values within the permitted limits. Selecting a larger number cannot demonstrate better wear performance or compatibility with your engine.
Match both the SAE grade and the required specification. If a product data sheet gives HTHS, check the test method and units before comparing values. Equal SAE grades can contain different measured HTHS values within the permitted limits. Selecting a larger number cannot demonstrate better wear performance or compatibility with your engine.