Low Oil Pressure

[bluebird]

I have a 1984 300SD, the oil pressure has always been at 3 while driving, 2 at idle, now it is at 3 when I first start it and as it warms up drops to 1 1/2 bars. I pulled the wire off the sending unit, grounded it and it pegs the guage. I replaced the sending unit with an unknown one off a parts car that had been run out of oil and water. They both behave the same 3 at startup and 1 1/2 bars when it starts to get warm, the temp guage hasn't even started to register yet.
I measured the resistance of the old sending unit = 8 ohms and the replacement measured 28 ohms. The original one rattles the replacement does not.
Do any of you know what the resistance should be on a NEW one?
Oil is Mobile 1 syn 5w40 and that has been in for 1000 miles behaved OK with 3 bars.
Any Suggestions?
Gary
OBK 37