^^I slightly disagree. My theory is still that changing the orientation will help get his pressures up. Mounted upright/sideways, the oil is forced thru the entire core on its way in/out. Depending on the internal design, it could be filling up with air.
I did some checking yesterday and this morning- With OG stock pump/gear and standard S13 pickup & large oil pan/cooler, my pressure at idle (warm) is about 1.2bar (17psi) and about 3bar (44psi) at 3,000rpm cruise on the freeway. I use 10W30 oil and right now, ambient temp is in the single digits/low teens. Again, being an SR; it could be apples and oranges... but hopefully it helps you get an idea.
Adding the extra fluid involved with a cooling system 'should' result in a pressure drop- but not like you are experiencing. If I were near you, I'd be happy to help you reorient the cooler just to see what happens. Good luck~
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