My friend the MB mechanic had a similar complaint. Tried changing the sending unit, but didn't fix it. Oil pressure is indeed VERY low at idle.
I'll check again, but I believe the cure was to replace the oil filter housing. This holds the pressure relief valve, and he was thinking there was something in it, or the valve or seat was scored, allowing enough oil to leak by at idle to drop the pressure to nearly zero.
I'd rather spend the money on a new relief valve than on mains!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!