I've heard that oil can turn into a thick, jelly-like substance in the crankcase at lower temperatures. Why does this happen, and what does this do to the engine?

Gelation Index

Of the two forms of pumpability problems discussed under Pumping Viscosity, the gelling tendency causing air-binding conditions is of most concern. This is when the engine oil becomes a gelled mass in the crankcase under particular cooling conditions -- cooling conditions that are not necessarily very low in temperature. Under these conditions, the engine can be started relatively easily. However, when the pump tries to pull oil in, a hole forms and the pump draws air rather than oil, creating an air-binding condition. It is thought that many cooling cycles can produce gelation. Fortunately, the cooling cycles are not very common, although some oil formulations are more susceptible than others.

The Gelation Index is a number indicating the oil's tendency to form a gelated structure at temperatures below 0°C (32°F). Gelation Index values above 6 indicate some gelation-forming tendencies. Numbers above 12 are of concern to engine makers. Numbers above 16 are known to be associated with oils that have caused engine failure.

An oil's Gelation Index is determined by using ASTM test method D5133. A tube of oil containing a rotor driven at 0.3 RPM is slowly cooled at 1°C per hour for about two days. This technique allows both the viscosity and any gelating tendency to be continuously measured. Data is collected throughout the cooling cycle and then analyzed to determine whether gelation is present and how severe it is.

Specifications set by ILSAC (International Lubricant Standardization and Approval Committee) have established that the Gelation Index of an oil should be 12 or less when evaluated in the temperature range of -5°C (+23°F) to -40°C (-40°F).