Originally Posted by Hans
I dont fully understand the correlation between trans temp and oil pressure but its related is all I can tell.
First, the hotter the oil, especially at temperatures above the thermostat set point (180-240F), the thinner the oil and the lower the oil pressure. This is because thinner oil flows more quickly and with less resistance around the bearings in the engine. Even though multi-viscosity oils are supposed to get thicker as they get hotter, and that may be true comparing flow at 0*F to flow at 160*F, I have not observed any engine running multi-viscosity oil whose oil pressure increases between 180 and 240F. Rather, I have consistently observed the opposite.
The transmission fluid is cooled in the radiator giving up its heat to the coolant that circulates to the engine. So, the hotter the transmission fluid, the hotter the coolant, and the hotter the engine. That'* why a separate transmission cooler lowers engine temperature. The separate cooler is able to remove heat from the transmission fluid without transferring much of it to the engine coolant circulating through the radiator.
So, cooler transmission fluid -> cooler engine coolant -> cooler engine temperature -> thicker oil -> higher oil pressure.
Just to be clear, we are talking about engine temperatures above the set point of the thermostat and engines whose cooling system capacities exceed the engine'* cooling needs.