You're right, the straight weight
oil should do nothing for hot conditions. The split grade oils have a higher viscosity index (around the 150's) as compared to the straight weight oils (in the 100's).
I think the whole point behind the multi-vis is the cold "drag" losses. It provides easier starting and less losses when the
oil's cold. At the same upper temp, the viscosity is the same as the straight weight.
Seems to be an issue with supply and demand. The closer the flow potential equals the flow from the pump, the lower the pressure. When revving, there should be little change in flow potential as the orifices and drain holes stay the same diameter (the only increases are from the increased speed of the bearings, cam, etc.), but the flow from the pump increases significantly.
I'd say that a new pump would do the trick. One question that comes to mind is - do these pumps have a relief circuit? Or can you (in an extreme case) put all kinds of restrictions with a high volume pump and run at 250 psi?