20w50 should result in a higher oil pressure, I would recommend that you try that and see what the oil pressure reads if it is approved for the climate you are operating in.
When you turn the steering wheel at a stop, do you also notice a slight drop in RPM? With my C280 with the M104, if I turn the wheel at a stop, there is so much drag on the engine from the power steering pump that the RPM drops. The ECU then corrects the slight RPM drop, but after a small delay. If the RPM does drop with the V8 engine as well, there is the possibility that it is due to the momentarily lower engine RPM and thus lower RPM of the oil pump.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record