My experience with Mobil 1 15W50 in high milage MBs is very good -- niether the 300SDL my brother has nor the 87 300D used any before the switch, and niether does now. I've not yet managed to get the oil changed on the 300TE, but expect it to use about the same amount after as it does now -- about a quart in 1000 miles (mostly front cover leak, I think).
Some folks have reported various amounts of new leakage after the changeover, though.
Both cars run a little smoother and start nicely on the synthetic, so I will stay with it.
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!