Everyone see Tom's post last week about pour point?
Quote:
"Is there any "formula" for converting an oil's pour point to actual usage temperatures?"
I would not trust any such formula. Pour point measures the temperature at which the oil stops pouring under its own weight, that is, under low shear conditions. In the engine, oils are sucked and pumped, which is much higher shear conditions and does not correlate to pour point. This is why the specifications call for CCS and MRV viscosity measurements instead of pour point.
Imagine a bowl of Jell-O. If you tilt the bowl, the Jell-O will wobble and perhaps sag a bit, but it does not flow out of the bowl like a liquid. This lack of flow would suggest that the Jell-O is a solid that would not flow where needed. Now take a spoon and stir, and the Jell-O will move fairly freely under the force (shear) of your hand. Then take wide straw and suck the Jell-O - again the Jell-O will flow up the straw under this vacuum force, but it will not flow back into fill the hole you sucked out. In other words, applying a force to an apparent solid material can cause it to flow and pump, even though it cannot do so under its own weight. The reason is that the Jell-O has a weak crystalline structure that breaks easily under force (shear) and reverts back to a liquid like substance that can be easily moved.
A similar situation exists with motor oils since mineral oils have waxes that grow crystals under certain temperature conditions, causing a "freeze point" as opposed to a "pour point". The difference is that "pouring" stops when the viscosity rises to a point that the oil is just too stiff to flow, while "freezing" occurs when the crystal structure from the waxes "knits" the oil into a weak solid, sort of like Jell-O. Crystal growth in oils requires a very slow cool down to occur, often with a pause or soak period. The pour point test cools at a relatively fast rate that can "super cool" the fluid, that is, it whizzes (technical term) right on past its freeze point and runs to its pour point, missing any freezing along the way.
The CCS test stirs the oil (applies shearing force) during the cool down and better simulates the shear rates of the oil pump than a simple pour point. The MRV test cools at a very slow rate with less shear and catches the effect of any freezing tendency.
Back in 1981 Quaker State had an oil that caused over 1,000 engines to seize due to these effects. The oil had a good pour point and CCS viscosity and could be readily sucked up and pumped by the oil pump when cold. However, their VI improver caused crystal growth under certain cooling conditions, turning the oil into a Jell-O like consistency in the pan. Then when the pump sucked the oil up from the reservoir in the pan, it created a hole and the oil was not able to flow back in and fill the hole. The pump then sucked air and the engines seized within minutes from oil starvation. This freezing phenomenon was prevalent and well documented in the Sioux Falls area where the temperatures during the failures cooled very slowly and paused for a while at about +10-15F. When simulated in the lab, the otherwise passing oil exhibited a freezing tendency. This temperature profile was referred to as the "Sioux Falls Cycle" and formed the basis of the cooling cycle used in the MRV test, which was then added to the J300 spec. QS owned up to the problem and paid the claims.
It's not the end-all, be-all.
