What might cause the engine to be difficult to start when the outside temperature is cold?

Starting Viscosity

The starting viscosity (or startability) of an oil determines how easily the oil will allow a cold engine to turn over and start at lower temperatures. The lower the viscosity and SAE 'W' grade at that temperature, the easier the engine will start. This is particularly true with diesel engines.

Modern gasoline engines with electronic ignition and fuel injection start more easily at low temperatures, making starting viscosity less of a concern. However, this leads to another more critical concern - the engine starts but the oil cannot be adequately pumped. This is discussed further under Pumping Viscosity.

To measure the starting viscosity of an oil, ASTM test method D5293 is used. In this test, a steel cylinder, or 'rotor', is inserted into a relatively close-fitting copper cup, or 'stator', filled with the oil in question.

The oil is cooled to a specific temperature at which point a certain amount of energy is applied to the rotor to turn it against the resistance of the oil. The rotor speed is measured to determine the engine oil viscosity at the chosen temperature. The unit of measurement for reporting starting viscosity is centipoise (cP). Centipoise is the amount of force necessary to move a layer of liquid in relation to another liquid. It is the standard unit of measurement for fluids of all types.

SAE has set specifications for starting viscosity associated with the 'W' grade of an oil as follows: