It sounds to me like you are worrying about normal oil pressure behavior. Oil gets hot, thins out, pressure drops. Power steering engages, engine RPM's drop for a second, oil pressure drops. What you need to watch for is a sudden drop in pressure at equivalent conditions and of course listen for any engine noise.
20/50 is probably a good bet for the warmer months. At least it will make you feel better. I have never been able to get an answer to whether or not it improves lubrication.
I would stay away from synthetics because of expense and potential leaks.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe