I found this very interesting article while doing some research on my own thread/post here. It gave me a better understanding of oil viscosities, and motor/engine oil in general.
I think it is time to go over passenger car automotive engine oils in detail. I will be writing several articles to be published soon so I will try to get some of it out here. I feel this is a very general topic for all car owners on this board.
www.minimania.com
From the write-up:
"It is time to introduce the concept of lubrication. Most believe that pressure = lubrication. This is false. Flow = lubrication. If pressure was the thing that somehow lubricated your engine then we would all be using 90 grade oil. Lubrication is used to separate moving parts, to keep them from touching. There is a one to one relationship between flow and separation. If you double the flow you will double the separation pressure in a bearing. The pressure at the bearing entrance is irrelevant.
In fact the relationship between pressure and flow is in opposition. If you change your oil to a thicker formula the pressure will go up. It goes up because the resistance to flow is greater and in fact the flow must go down in order for the pressure to go up. They are inversely related. Conversely if you choose a thinner oil then the pressure will go down. This can only occur if the flow has increased."
Bearing "separation pressure" doesn't depend on the flow through the bearing - it depends on the physical dimensions of the bearing, rotational speed and the oil viscosity which determines how much the oil wedge squeezes down in use (the MOFT), and therefore the resulting pressure in the separating oil MOFT wedge. A journal bearing will only flow what it's physical dimensions, rotational speed and oil viscosity will allow. On top of that there is some added flow through the bearing due to the positive displacement pump supply pressure at the bearing entrance which doesn't effect the MOFT, but does effect the flow beyond what the bearing wants to naturally flow due to dimensions and speed, which can help reduce the temperature rise in the bearing.
With a positive displacement oil pump not in pressure relief, the flow basically stays the same at any given engine RPM, so there is no situation where the "flow must go down in order for the pressure to go up". No matter what the oil viscosity is, any time the flow goes down the pressure must also go down. If you have a thinner oil, the pressure goes down because there is less resistance to move the same oil volume through the same circuit ... it doesn't go down because the "flow has increased". If the flow did increase at constant X viscosity, then the pressure would have to also go up. The pressure will go down if the viscosity goes down with the flow held constant.
