VIIs are coiled up when cold but uncurl with heat and the VII polymer expansion causes the oil to have a higher viscosity. These viscosity modifier polymers are relatively large molecules compared to the base oil molecules. VIIs are polymer additives in modern engines oils that raise the Viscosity Index of the base oil. HTHS can be thought of as a measure of the Viscosity Index Improver (VII) capability of an engine oil. Generally, the higher the HTHS rating of an engine oil, the better the protection of journal bearings. Standard SAE J300 places minimum requirements for HTHS for each viscosity grade of engine oil. Within highly loaded bearings, engine oil has a temporary loss in viscosity and ASTM D-4683 is test method which is thought to be representative of the condition encountered in the bearings of automotive engines in severe service.
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