Here's a useful tool
http://www.shellglobalsolutions.com/base_oils/toolbox/blending_pop.asp Just plug in your reference visc @ temps, then plug in the new temp ..voila~
The automotive oil pump is positive displacement. If the volume at a given viscosity generates a resultant pressure in excess of the pressure relief setting, flow is shunted to either the suction side of the pump ..or back to the sump.
