After starting my engine in colder weather, why is it a good idea to watch my oil pressure indicator light and let my engine warm up for a few minutes?

Pumping Viscosity

Once an engine has been started, it needs a continuous and adequate supply of oil. The pumping viscosity (or pumpability) of an oil determines how easily an oil will flow from the oil pump to critical parts of the engine in cold temperatures. Sometimes, even if the SAE grade is correctly chosen, this flow isn’t sufficient or no flow occurs and engine damage results. The oil pressure indicator is the only safety for this condition and engine warm up is a protective approach.

There are two types of pumpability problems that are caused by the engine oil. One type is called flow-limited behavior and the other, more unpredictable and critical is called air-binding behavior.

In the first case, if the oil is too viscous (thick) to pump at a sufficient rate to satisfy the engine, flow-limited behavior occurs. This is where the oil will not flow rapidly enough to prevent progressive damage to the areas needing lubrication during low temperature operation. A flow-limited oil may not be adequately pumped because of having too high a viscosity at the low temperature at which the engine is started.

To measure the pumping viscosity of an oil, ASTM test method D4684 is used. In this test, a rotor is inserted into a stator containing the oil sample and very slowly cooled at a constant rate over approximately a two-day period until it reaches a specific temperature. A small force is then applied to turn the rotor and the rate at which the rotor turns is related to the viscosity. The unit of measurement used for reporting pumping viscosity is centiPoise (cP).

On the other hand, if the oil, which would normally flow at a low temperature, has instead formed some kind of gelated structure (think gelatin), a condition known as air-binding or gelation occurs. Under these circumstances the engine fails to pump the lubricant it requires from the gelated oil in the sump and becomes air-bound. In this case, severe engine damage occurs within a few minutes. This condition is discussed further under Gelation Index.

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