I agree about the RPM drop from the PS pump leading to oil pressure drop. Probably nothing to worry about. However, turning your steering wheel while parked isn't particularly good for your tires, nor I suspect the PS pump, especially at the extremes.
I personally would HIGHLY recommend synthetic oil in this engine. The M119 engine runs hot and I like the extra insurance synthetics provide at high temps.
If you switch to Mobil 1 15W-50, I assure you your idling oil pressure will rise. I don't know if that actually matters in the overall scheme of things (the owner's manual tells you idle oil pressure as low as 0.3 is OK and "will not jeopardize operational characteristics") but it certainly makes me happier to see a higher pressure. Mine used to drop to 0.5 at idle in summer, and after the switch to Mobil 1, always stays about 1 bar. No leaks either, and I switched at 80,000 miles.