The viscosity of engine oils (and all fluids) changes with temperature. The measure of a lubricant's change in viscosity with respect to temperature is its viscosity index (VI), which is an arbitrary scale. It was originally based on the viscosity changes of the best and worst mineral oils of the time, with VI=0 having the greatest change in viscosity and VI=100 as having the smallest change in viscosity for the same change in temperature. The viscosity index today is based on viscosity measurements at 40°C and 100°C.
Viscosity Index Improvers (VIIs) are added to oils so that the oil performs like a low viscosity oil when cold and a higher viscosity oil when hot. A VII is a polymer additive that is coiled up when cold but uncoils with increasing temperature. For this reason, low weight (like 5W and 10W) base stocks are used in multigrade conventional oils. Because of the molecular nature of VIIs, high shear (as found in meshing gears) and high temperatures (as found at the ring grooves) can break down the VII molecules so that the oil loses its viscosity.
High-Temperature, High-Shear (HTHS)
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. Standard SAE J300 places minimum requirements for HTHS for each viscosity grade of engine oil. Generally, the higher the HTHS rating of an engine oil, the better the protection of journal bearings. HTHS can be thought of as a measure of the VII capability of an engine oil. The minimum HTHS for a all 20-weight oils is 2.6 cP while the minimum HTHS for a all 30-weight oils is 2.9 cP. Starting with the the J300 NOV2007 version, for winter 40-weight (0W-40, 5W-40, 10W-40 grades) oils, the minimum HTHS is has been raised from 2.9 to 3.5 cP. For all heavier grades of oil, including HD 40-weight (15W-40, 20W-40, 25W-40, 40 grades), all 50-weight (0W-50, 5W-50, 10W-50, 15W-50, 20W-50, 25W-50), and all 60-weight (0W-60, 5W-60, 10W-60, 15W-60, 20W-60, 25W-60) oils, the minimum HTHS is 3.7 cP.
When selecting an engine oil for winter use, the owner's manuals of early cars would specify something like: During winter, all oils used should have a pour point or cold test below the lowest anticipated temperature that will be encountered during its use. The pour point of an oil is the lowest temperature at which it will still flow. This was an issue with early engine oils because the oil would contain paraffin wax which would start to crystallize at low temperatures. Pour Point Depressants are additives that inhibit the formation of wax crystals. See Pour Point Measurement.