Change the oil and filter again at 2500 miles after you first change to Mobil 1 -- all the crud it will wash free in the engine will nearly plug up the filter! Once you get the crud out, change it as often as you feel you need -- I run 5000 or so miles between changes (factory recommendation for the Volvo is 7500 with a filter change every 15,000! -- I change the filter every oil change.)
The 420 is a wonderful car -- decent milage, plenty of power, and that lovely W126 chassis!
If you start to get a puff of blue smoke on startup and some oil consumption, get the valve guide seals replaced -- oil isn't good for the combustion chamber! They usually get hard and start to leak at about 150,000 miles or 10-12 years.
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!