Originally Posted By: Tom NJ
These kind of cold pour tests are measuring the oil's ability to flow under its own weight, sort of like a homegrown low shear viscosity test. This property is important because if the oil can't flow, the oil pump pick-up will suck a hole in the sump oil that will not refill, causing the pump to suck air and leading to oil starvation. Thousands of engines seized back in the early 80s by this mechanism from an oil using a VI improver that gelled the oil under certain temperature cycles.
These homegrown cold pour tests, however, are not well controlled, and even if they were do not tell the whole story. The oil pump will move even a gelled oil through the engine because it creates shear that breaks any weak jelling structures from residual waxes. Think of Jello - it is a weakly jelled liquid that behaves like a solid, but you can still suck it up through a straw by applying vacuum to the top of the straw. The straw, however, leaves a hole in the Jello because it cannot flow under its own weight to refill the hole, and you wind up sucking air. So long as the oil flows and refills the hole left by the sucking action of the oil pump, the engine will be lubricated.
The Cold Crank Simulator (CCS) is a high shear test run on motor oils to confirm that the pump can move a cold viscous oil sufficiently, and the Mini Rotary Viscometer (MRV) is run to confirm that the cold sump oil will flow back into the hole under its own weight. These two tests are much more telling with respect to an oils performance at low temperatures than simple uncontrolled pouring tests, and they are required by the API specifications, while a pour point is not.
Tom NJ/VA
Well said. The cold pour is a part of the picture, but not all of it. The straw in jello is a great analogy.
Also you have to remember that the oil still has to drain back down to the pan once pumped, so the oil pump may be able to suck the pan dry if the oil does not drain back down on it's own fast enough, which is exactly what the cold pour tests show.