I am pretty sure your oil pressure gage readings are normal for a fully warmed up car. I know the German manual for the W123 Diesels says 0.3 bar at idle with the car fully warmed up is ok, provided the meter pegs when the engine is under load. All my cars, except my 190E 2.3-16 go below 2 bar when idling fully warmed up. The 190E 2.3-16 always stays pegged at 3 with the engine running. But that is the only car I have seen that never drops off the peg when warmed up. Sounds like your idle lowers slightly when your A/C unit comes on.
Good luck, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)